2. for which bit is for which channel - для какого разряда какой канал. ?

Контекст не надо переводить:
Multiplexing is executed in synchronization with the octet signal pulses, as seen in the next figure. Bits corresponding to specific channels have specific position in a 8-bit periodical sequence, between two consecutive octet pulses. In this way, the chip is properly informed for which bit is for which channel.