My understanding is there's 3 tiers; genuine, OE, and OEM. OE being the exact same part as a genuine one, only difference being the missing BMW stamping/etching.
OEM parts are produced by manufacturers that produce parts for BMW, possibly in other applications and not necessarily the part in question.
FCP has an article that explains this:
https://blog.fcpeuro.com/the-great-d...vs-aftermarket