Single-level and multi-level BOMs

A single-level BOM lists only the parts that go straight into one item. A multi-level BOM keeps going down, listing the parts inside each sub-assembly too.

Same product. Two ways to look at it.

Single-level: one step down

A single-level BOM shows an assembly or sub-assembly with only one level of children.¹ A child here just means a line directly beneath its parent.

Say you build a garden gate. The gate's single-level list might read: frame, latch, hinge set, paint. It doesn't tell you what the frame is made of. For that you'd open the frame's own list.

This is handy when one person is looking at one build step. You see the parts for that step and nothing else.

Multi-level: the whole tree

A multi-level BOM lists the assemblies, components and parts needed to make a product in a parent-child, top-down way.¹ A sub-assembly can have its own children, and those can have children too.

The ERPNext manual puts it as each item having its own BOM, which forms a tree of items with multiple levels.²

You'll often see this called an indented BOM. It puts the highest-level item closest to the left margin and indents the parts used in it further to the right.¹ On screen it looks like a folder list.

How the software links the levels

You rarely type the lower levels again. In the ERPNext manual, when a line is a sub-assembly, the default BOM for it gets fetched.² So the frame's list is written once and reused wherever the frame appears.

That matters when you change the frame, because there's one list to fix instead of many. Other tools may handle this differently, so check your own tool's help pages.

Explode and implode

Two terms turn up in reports. An "explosion" breaks apart each assembly or sub-assembly into its component parts.¹ An "implosion" goes the other way, from a component up to the assembly it feeds.¹

In plain words: explode answers "what do I need to build this?" Implode answers "where is this part used?"

The ERPNext manual has an exploded table too. It lists the raw materials for the whole product and also fetches the raw materials for the sub-assemblies and phantom items.² A phantom BOM is only a logical grouping of raw materials.² The plain table doesn't go down into the sub-assemblies.²

Which one do you need?

If you make a few simple items from bought-in parts, a single-level list may be all you need. If you build things in stages, you'll want the levels, because you'll build and sometimes stock the sub-assemblies on their own.

Don't flatten a real sub-assembly into loose parts just to save typing. You lose the chance to change it in one place.

For what each line in these lists holds, see What a bill of materials is and what it holds.