5.1 vs 7.1

What happends to the audio signal from a 5.1 source when you have a Pre/Pro set up for a 7.1 system. Does the 5.1 signal get sent to the Side Speakrs or to the Rear speakers of a 7.1 set up?
The side speakers.
It depends on what DSP settings you have. If you have a "straight" type setting (no DSP), you get sound from the surrounds but get nothing from the rears unless the source is 6.1 or 7.1. If you want the rears active all the time, most people use a PLIIx or PLIIz setting, which will separate the back speaker sound from the surround channels. It separates the sounds rather than just duplicating them. I find PLIIx better than trying to duplicate the surround signals to the rears. PLIIx has a movie, music and game mode.
That answers my question. Thank you very much.