The letter/digit-space is per GDS, not per-airline/industry, however different airlines can have particular additional restrictions on codes. Some airlines don't allow 0 and O (i.e. Qantas), others don't allow I and 1, and others always/usually have the same ending letter (unless code-sharing).
Another thing to note is (unless something's changed in the 15 years since I left the airline industry) that non-trivial booking changes (i.e. not just passenger information changes) generally result in a new PNR being generated to replace the old one, effectively using up the old code.
