This is binary math. Instead of ones, tens, hundreds... you have ones, twos, fours. This is also zero-based.
So:
---------------------- 4 2 1
Ch 1. off off off == 0 0 0
Ch 2. off off on == 0 0 1
Ch 3. off on off == 0 1 0
Ch 4. off on on == 0 1 1
Ch 5. on off off == 1 0 0
Ch 6. on off on == 1 0 1
Ch 7. on on on == 1 1 1
Cheers,