There are many reasons that users save assemblies as part
files, one of which is to create a simplified version of an assembly (hide
internal, small bolts and fasteners, and unnecessary components) that can then
be utilized in another assembly (i.e. another top-level layout assembly) to
facilitate other workflows, especially to improve performance and limit the
number of documents files required.

Saving an Assembly as a Part now preserves geometry
references (internal ID’s) of the body/component topology in the newly saved
Part file so that later if changes are made to the original assembly file, a new,
simplified Part file can be saved and replace the old using the Replace
Component command, preserving any references in the assembly such as Mates
which would have otherwise failed (any items that were deleted/removed from the
original assembly will still fail).

The Custom Properties are now transferred to the saved part
file to better facilitate the top-level B.O.M. criteria as well as inherit the
configuration name from the parent assembly. In addition, a log file is saved
with other important information from the parent assembly within the Design
Binder if more information is desired or needed.

In Beta 2, this will be optional when saving to a Part file
in the form of checkbox shown below. Please give this a try.