pyinstaller/pyinstaller — comment on commit 3482ea7a493de9a3a378fd608428cad8c8c3e618
File: PyInstaller/depend/modules.py line 136
By rasky:

Generically speaking, you can have a custom spec file which packs a subset of Python files into a PYZ file, and then drop it on the disk somewhere. If you add the PYZ file to sys.path, PYZOwner will parse it and make the modules within it available at import time.

It's not the most used feature, but I don't see a reason to drop it.