Assembly Expansion BOM properties allow users to determine which items they want to show on their BOMs with a few rules that users can apply. Users can choose to have their assemblies be treated as a single part when entered into the BOM with the Treat as -Part option inside of the Assembly Expansion in BOM. This option can be applied to all link assemblies, so only the total number of assemblies are counted. This tool is great for cases where you reuse the same assemblies like fasteners that are all standard.  This tool has functionality that works on multiple assembly levels, so you can choose which assemblies you would like to be treated as a part and won't be included inside of the BOM.

In the example, we have the main assembly with two sub-assemblies beneath it.

Each of these sub-assemblies contain a body and fastener assemblies underneath them, which is seen when we create the BOM for this scene.

We can then go ahead and edit the properties of the fastener assembly and treat as part for all linked instances.

This will change the BOM to only present the # of assemblies rather than the total parts within those assemblies.

If we were to take a sub-assembly and change the BOM properties to treat as part, that means that the fastener assemblies located beneath the sub-assembly will not be counted towards our BOM.

As you can see, this tool is a simple solution that allows you to include only the information that you need on your BOMs, and is a simple process that you can get to by right clicking an assembly. Lastly, take this capability a bit father when working with External link assemblies. For example: In your external assembly file and its associated drawing, you may want to have the BOM account for all the parts in the assembly.  When you insert this assembly into another design however, you may want to treat "Only This Assembly" as a part so that it is recorded as a single line item in that files BOM.

Fasteners within their own assembly:

Fasteners when inserted into another Scene file and edited the Sub-assembly level to Treat as Part:

This is a powerful tool that helps build a expanded and collapsed BOM based on where a file is used and for the desired BOM's for manufacturing for each case.