Its pretty Simple, Only official Apple store can have genuine apple parts, which includes glass/digitizers. If someone is selling you a digitizer and telling you its OEM and its not an official apple store, or at lease an authorized apple store then its more than safe to assume the glass you are getting it a generic version, not an actual product made in apple factories. Saying that, I've used generic glass and found it works just as well as original, but that's just my experience.