In the previous window we selected the 'Mouse' from the inventory list and the Parts List Editor pops open showing a list of all the parts we need to build our mouse.

Highlighted in Yellow is the 'top level' assembly. Directly below that are 'Sub Assembles indented to the right indicating that they're contained inside the assembly directly above.

There's no limit to the number of items a parts list can contain and, as you see above, an assembly can contain other assemblies.