With 3DVIA Composer you can create a top-level BOM in a few steps.
1) Select the assembly
Use Select Actors and set the selection to Child Level 1 the pick Select
This will select the Assembly as a top-level selection (sub-assemblies will appear blue in the tree).
2) Set the BOM Workshop
Under Options, Definition tab Set the settings as shown
Next set the options for BOM IDs and pick Generate BOM IDs. If you also want Callouts then select Create Callouts at this time also.
3) Save your view
The BOM is a top-level BOM with sub-assemblies and parts.
Callouts for sub-assemblies are black by default. You can select the callout and change this if desired.