On the website it says:
About Check-in online
Fly with Etihad and enjoy a hassle-free journey. Provided your flight is operated by Etihad Airways (not a codeshare partner), you can check-in online from 24 hours to 2 hours before departure.
What do they mean with (codeshare partner)?
And on website:
Form of Identification
The following forms of identification can be used:
PNR (Booking Reference Number): 6 digit alpha-numeric booking reference (Etihad Airways PNR) located on your Itinerary. (e.g. YIJHDZ)
Etihad Guest Number: 12 digit number ( e.g. 100013753546)
The only codes I am seeing on my confirmation tickets are:
Main Booking reference: OCL***
Airline Booking reference: BLQ***
Ticket number passenger: 607**********