Key takeaways
- A bill of materials (BOM) is the list: what components and quantities are needed to build one unit of something.
- Kitting bundles already-finished, separate items together for sale or shipment — the components stay distinct, just packaged as one unit.
- Assembly transforms components into a new physical item, often permanently joined or fabricated — and it is driven by a BOM.
The short version
These three terms get used interchangeably, but they describe different layers of the same process. A bill of materials is the list of what goes into something. Kitting and assembly are two different ways of actually turning that list into a physical outcome — and the difference between them is whether the components stay distinct or get transformed into something new.
- Bill of materials (BOM): the recipe — components, sub-assemblies, and quantities needed to build one unit of a finished good
- Kitting: bundling separate, already-finished items into one sellable or shippable unit, with no transformation
- Assembly: combining components into a new physical product, often permanently joined or fabricated, guided by a BOM
How the three relate to each other
A BOM is not optional for either kitting or assembly — both need a list of exactly what components and quantities are involved before stock can be deducted correctly. The difference between kitting and assembly is what happens physically to those components once the BOM is executed.
In kitting, a kit is essentially a sales-oriented BOM: three individually sellable items get grouped and sold together as a bundle, but each component remains the same distinct product it was before — a returned kit can simply be "unkitted" back into its original parts. In assembly, components are combined into something that did not exist as a separate SKU before — a finished bicycle from a frame, wheels, and a drivetrain — and that transformation is not generally reversible the same way.
Which one applies to your business
If you are grouping products that are also sold individually — multi-packs, gift sets, subscription boxes — that is kitting. If you are physically building a new product from raw materials, purchased parts, or sub-assemblies that are not sold on their own, that is assembly, and it needs a full BOM (often multi-level) behind it. Many businesses need both: assembly to build a finished good, and kitting to later bundle that finished good with accessories for a promotion.
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