I am 99 44/100% sure that you are not choosing your own PNR on KE, only another reference that the res can be located with. I have been in IT architecture for 30+ years, and we'd never let a user pick the key for a record. It's just user experience suicide. The earlier posters with 'Your Number is Used', have it right - keeping millions of records unique requires some sophistication.

For me, the ability to have them all tied to my FF program number is sufficient. I can look them all up at once, and in a pinch so can anyone at the airport. I make sure everything has that one reference, and I'm covered.