I find it interesting that you can have a ticket without there being availability...
Normal Customer
Holding (e.g. via a travel agent, ticketing deadlines to the side for a minute) a "seat" from a fare class is free and easy. You say take 1x M class seat. No cost. The ticketing bit confirms and pays for the seat/reservation. You end up with availability + a ticket = you travel.
Staff Travel
You purchase a ticket without there being any availability. A 125 ticket does not mean you are travelling!(to a normal customer a 125 ticket number is critical!). The availability is found and (not always!) added after.
Well it’s called standby for a reason... plus there’s always jumpseats!