It depends on the age of the vehicle.
It might be Service and Secondary but Primary and Secondary circuits are normal descriptions, however, it isn't unusual to have two primaries as well as a secondary.
Typical Air-over-hydraulic System.
Modern vehicles have dual-circuit systems which might split the hydraulics as well as the air, thus reducing the possibility of total failure, so with
one circuit failure you would still have adequate (if reduced) braking available.
The secondary circuit must achieve certain parameters - and might be applied through the 'parking' brake.
Many people overlook the handbrake as being available when the wheel brakes have failed, but it will usually be certified as a secondary brake application system.