I have been out of the loop for a while, this is probably very old news but I will post it anyway.
Any apps that pull UDIDs from iOS devices now return FAKE UDIDs, and have done so since iOS7 I suppose.
Any UDID starting with ffffffff… is fake.
I had a few testers give me their UDIDs from app store apps, such as udid sender. Apple, the generator of the fake UDIDs, accepts these fake udids in the members portal and uses up a slot for the year of your 100 max UDIDs. A generic error is given when trying to install:
Unable to download app - “appname” cannot be installed at this time
I think the fake UDIDs were introduced to facilitate privacy. It’d be kind of annoying if you uninstalled Facebook, then reinstalled it later and it automatically logged you in. A fake UDID is still useful for tracking the lifetime of an app install, though.
If you have testers who are willing to install Xcode (7+), Apple has now lifted the restriction where you had to have a paid developer account to install apps on your own phone. So you shouldn’t necessarily have to have your testers use any of the slots on your paid dev account.