We had codes that were only good for a certain time limit, to log in and register for classes.
Which group got to register first rotated by semester/quarter/trimester
Fall trimester: A-F, G-R, S-Z
Winger trimester: G-R, S-Z, A-F
Spring trimester: S-Z, A-F, G-R
and the boundaries always shifted a little, usually around D-E-F-G, L-M-N and R-S-T, due to perhaps more Smiths this semester than last, etc
If we missed the time the code was valid for us, then we had to wait until the end to get the free-for-all time slot