What Signals are on the Outputs?
Mix-L and Mix-R

This is the regular master output in ReBirth RB-338. These are the only stereo channels, all other channels are in mono.

If none of the other channels are used, then this carries all the sound from ReBirth.

Signals that are activated separately are removed from this mix.

If for example the 909-Mix channel is activated, then Mix-L and Mix-R carries all the sound from ReBirth RB-338 except the 909, which will appear on its own channel.

The individual outputs are described more closely in the ReWire chapter of the ReBirth manual.