there are plenty of “empty” fields in GSM specifications, in order to be future-proof ! That’s why GSM went from phase 1 to R’6 without major changes in the message flows, or in the messages themselves.
The fundamentals are always the same, and yet it’s still working competitively against 3G 🙂 (well, ok, i could limit myself and only say “it’s still working” 😀 )
Have a good week end !