A PNR, or booking reference, is on BA a 6 character combination of letters and numbers. Not all numbers and letters are used and it's not exactly randomly generated since there may be another PNR from someone else lurking around. Usually there is no question it is a PNR given the jumble of letters and numbers, but sometimes there are no numbers in the PNR and it looks like a word or abbreviation. That's quite normal - in my next 20 PNRs showing in my App, 5 of them have no numbers in them. I've had a few word like PNRs, and one that started WWW and so looked like a website address.

Those airlines on SABRE or derivative have no numbers in their PNR. Aer Lingus always starts with the number 2. It's just the way it is.