Hi Fazel,
If both “Front Transmission” (going from outside to inside) and “Back Transmission” are available, rmtxop will prefer the “Front Transmission” matrix. Otherwise, the “Back Transmission” matrix will be used after reversing its direction using reciprocity. Only “Visible” data will be used in either case. If your data does not have color information, as in most cases, then the coeffeicients you use for the -c option do not matter, except that they should sum to 1.0.