Each circuit in an out door unit has its own number, so how does the numbering system work?

We use the following number system for each circuit.

It is: 9 = 9,000 BTUH; 2 = 12,000 BTUH; 5 = 15,000 BTUH; 8 = 18,000 BTUH; 4 = 24,000 BTUH; 3 = 30,000 BTUH; 0 = An unused/empty circuit.

Each indoor unit has its own circuit, which is independently controlled. Evaporator styles and sizes may be mixed in over 600 combinations in two, three or four zone systems.