For information purposes:
In PT7, the usfm.sty file was shared with every project (as it is in PT8). However, many projects made custom modifications to usfm.sty which then got S/R’d to other projects. This created nightmares when different projects made different changes to the usfm.sty file - they would start fighting over which one would win.
In PT8, we decided to no longer have non-custom stylesheets be S/R’d. However, because projects had made changes to their stylesheets, we had to make sure those changes were not lost. So, during migration, Paratext will take the stylesheet used by the project in PT7 and compare it to the usfm.sty file that ships with PT8. Any differences that are found are put into a custom.sty file for that project. This is why some projects are seeing a custom.sty file in their project when they didn’t have one before.
(Note that any existing PT7 custom.sty file is merged into this new one - which is why the one project had a custom.sty.bak file.)
However, because of the problem in PT7 where any project’s custom modifications to usfm.sty could be propagated to other projects unintentionally, this can create a custom.sty for a project in PT8 that is not what the project intended.