For the record, info from a paramedic source is that there is no general requirement or designation for unit numbers. Services are allocated a block of numbers by MOH; based on their zone as some have already mentioned. How the services organize their units with those numbers is their own discretion.
I.e. York Paramedics:
30XX, 31XX, 32XX, 34XX are transport units. (No designation beyond this for single/dual beds or PCP/ACP truck)
33XX are rapid response units.
36XX are special response units.
37XX are supervisors.
3900 for the multi-patient unit.
But that's just how York Paramedics designate their numbers. You may or may not find similar patterns with other services. It just depends on how they roll.