The channel is assigned to a module by the operator and can be any channel desired. An address is a unique identifier assigned by the 18R2 to each module and other devices such as an Audio Box. Think of the address as a unique serial number for a device. Addresses start at A00 and increment up each time a module syncs to the 18R2.
Different from an 18M, when the 18R2 is synced to a 36M or 72M, the 36M skips two address, and the 72M skips four addresses.