Having faced the same problem, I think I can say why there is no mapping.
Channels are for data aquistion, so actually a DAC, ADC, DO-Driver ...
Terminals are used for timing (trigger) and internally routed. So a diffrent concept, and a diffrent circuit.
Actually the same goes if you use analog triggering, you will have a different ADC (I found out because I got only 10-bit on a 18-bit M Series Device).
Felix