It's only the channels that we want to assign, e.g. an AUX send that sends channel 1+2 of a standard stereo track to channels 3+4 of a 4-channel-bus. Or channel 3+4 of a 4-channel-track to channel 1+2 of a standard stereo bus.
For sending audio to multiple buses we would insert multiple AUX sends.
It's only the channels that we want to assign, e.g. an AUX send that sends channel 1+2 of a standard stereo track to channels 3+4 of a 4-channel-bus. Or channel 3+4 of a 4-channel-track to channel 1+2 of a standard stereo bus.
For sending audio to multiple buses we would insert multiple AUX sends.