Form / Fit / Function

What's the difference between version and revision? Should we create a new part or just create a new BOM revision? How do we track versions of the same SKU in inventory? These are all questions that individuals and organizations face through the design and production process. Luckily, there are established best practices to provide some guidance.

Aligni is one of only a few systems to combine PLM (product lifecycle management) and MRP (material requirements planning) capabilities into a single integrated system. While this integration provides countless benefits to data consistency and communication among interested parties, it means that Aligni must merge long-established rules and best practices of both PLM and MRP systems. This integration has previously lined the pockets of many well-heeled consultants and is typically fraught with varying levels of frustration.

One important consideration for PLM and MRP implementation is a part numbering system (as it is called on the PLM side) and SKUs or stock-keeping units (as they are referred to on the MRP side). The common terminology applied to the rule is “form / fit / function”.

Throughout the lifecycle of an assembly and its components, engineering and operations often need to determine if they should introduce a new part number or create a new revision of an existing part number. To inform this decision, the 3 F’s are considered:

  • Form – This refers to the relevant physical characteristics that define the geometry and structure of the part (e.g. shape, size, dimensions, weight).
  • Fit – This refers to how a part connects or interfaces with other components in a system. (e.g. mounting points, connectors, or other interface mechanisms). A proper fit ensures that the part can be physically integrated into the system without modifications to itself or surrounding parts.
  • Function – This refers to what the part does — its operational characteristics and performance specifications. The function includes the part’s capabilities, such as its power rating, operational parameters, or software compatibility. A substitute part must fulfill the same function to perform as expected within the system.


Formulation is an often considered fourth dimension for the compatibility of an item within a system context. This is typically applied within a more specialized context where the chemistry or pharmacology influence system compatibility.

For example, in material science and engineering, the formulation of materials affects the thermal, mechanical, and electrical properties of a component.

In the context of product design, formulation of materials can also affect the usability, durability, or sustainability of the system.


Let’s start with a bit of terminology. A manufacturer is the maker of something — a widget, a gear, a screw, an integrated circuit, resistor, etc. The manufacturer’s part number (MPN) is used to uniquely identify that product among all other products offered by that manufacturer. It is assumed that the manufacturer and MPN taken together uniquely identify the product throughout the world.

Within Aligni, a company assigns an internal part number (IPN or just PN) to each MPN in their database. This IPN is usually a shorter sequence of digits because the company is only concerned with a small portion of the parts in the world. Aligni’s IPN may also be considered interchangeably as a SKU (stock keeping unit).

Finally, an inventory unit is how Aligni keeps a record of a single part at some location in some quantity. So an inventory unit may describe one widget with a specific serial number or it may describe a reel of 7,597 capacitors. The inventory unit is just a record or “instance” of inventory.

The Rule

Form / Fit / Function Rule

When deciding whether to introduce a new part number or create a new revision of an existing part number, consider the form, fit, and function of the new item. If the form, fit, and function of the new iteration are identical to the old iteration, then a new revision is appropriate. If the form, fit, or function of the new item differ from the old item, a new part number should be generated.

This rule generally originates from PLM best practices, but the implication for MRP operations is that inventory of the same SKU or part number may be mixed. Simply put: do not represent revisions in inventory.

This rule is universal among PLM and MRP operatives. It’s a short wall between order and chaos. And yet, it is the cause of much consternation among PLM newcomers. This is perhaps because the engineers commonly using the PLM side track changes using revisions and feel that those revisions should be tracked in inventory just as well. However, if revisions are physically separated somehow in the warehouse, then the revision number might as well be integrated with the part number / SKU. And if this is the case, then we’re back where we started — but a new part number has just been generated!

If the revision information is not to be tracked in inventory, then what’s its purpose? Revisions are used for documentation. Design drawings, bills of material, engineering change orders, and other information form the design history of an assembly and are recorded in the revisions as it evolves. But for the purposes of inventory and production management, the revision information is irrelevant.

The lasting beauty of this rule is that it actually makes operations much simpler. To the production manager, there is no ambiguity about revision meanings in inventory: if two items are in the same bin, then they are interchangeable. To the design engineer, if the next iteration of a design will describe something that cannot be interchangeable in inventory, then a new part number must be generated.

Engineering Change Disposition

The engineering change process includes consideration of the “inventory disposition” when a change is ordered. This is intended to maintain consistency in inventory when a new revision is released. Here are a few typical disposition actions:

  • Use as-is – Existing inventory may be used as-is.
  • Rework – Existing inventory must be reworked to make it consistent with the engineering changes to the part. This will assure that old and new inventory will be form / fit / function compatible.
  • Scrap – Existing inventory must be scrapped.

Each of these dispositions provide a path to assure that everything in inventory is interchangeable once the new revision becomes active. Perhaps more importantly, dispositioning inventory provides critical documentation that this was a consideration throughout the engineering change management process.


Here are a few common situations to consider. Your organization may treat these scenarios differently based on processes in place or other requirements.

Manufacturer Acquisition

In the electronics industry, manufacturers are often acquired. In most cases, the manufacturer of a part changes but the manufacturer’s part number remains the same. In this case, it’s often easiest to simply change the non-revisioned parameter “manufacturer” to indicate the new entity. Aligni will log the change in the part’s history, but there is no need to issue a new part revision or create a new part. Items may be mixed in inventory even though the part markings will change with the new manufacturer marking.

Semiconductor Die Revision

It often happens that a semiconductor device undergoes a die revision. Microprocessors and memory may undergo a “die shrink” as the part is migrated to a newer fabrication technology or may have design defects repaired. In these cases, the manufacturer almost always issues a new part number because the parts should not be commingled in their inventory. Your organization may take a different position on this commingling, but we suggest creating a new part number. In the case of memory die shrinks, the form of the part has changed – it may be functionally equivalent, but power consumption is usually different. In the case of design defect fixes, the function has obviously changed, so inventory should be segregated. The relatively minor inconvenience of BOM updates to all assemblies using the item can serve as a helpful documentation event to track the change.

Note that the revisioning of assemblies using the part is a different matter and requires a different approach. In some cases, an engineering change request will be warranted to review the repercussions of the change. An engineering change order may specify a scrap inventory disposition to eliminate the defective or non-conforming material if an important defect is repaired. Even when the benign “use as is” disposition is specified, the change is documented for future reference and that alone is helpful!

Mechanical Changes

Mechanical parts are subject to the same considerations as electrical components. Since these are often designed in-house, careful consideration of the item’s form, fit, and function. This is where the engineering change management process can come in useful. Everyone involved in the design and production of the product can discuss all aspects of the change and establish a disposition that will be most effective.

BOM Changes

Depending on your organization and the nature of the change, a BOM change may or may not require a new part number. Consider a printed circuit board assembly (PCBA) and a change that swaps one component for another without any PCB changes, functional changes, or specifications. For most organizations, this change would create a compatible product and a new part number would not be required. However, a medical device manufacturer may have a regulatory requirement to keep the resulting PCBAs separate in inventory for traceability so they would need to issue a new part number (SKU).

Closing Comments

There is sometimes reluctance to creating a new part (item) as the result of a seemingly minor change to the design of that item. This reluctance is usually in the interest of saving some time creating a new item or maybe additional paperwork. Typically, however, this additional time is well spent as it reduces ambiguity and confusion among interested parties, be they suppliers, manufacturing, sales, customers, and regulatory agencies.

Your organization will need to consider what policies and practices to put into place to satisfy these interested parties within the context of your business.