There's no such thing as recording in 5.1 to my knowledge.

Sometimes 5.1 mixes are done by fans in post by mixing a SBD (front 3 channels) and an AUD (rear channels), or by mixing an FOB (front 3 channels) and an OTS (rear channels).

When pros record in 32 track for a concert, there are sometimes two or three pairs of AUD mics i think scattered about. So an engineer could run a pair of AUD mics at the SBD, or further back, and then in post use just this pair for the rear two channels, and use the rest of the SBD mix for the front three channels. That's the best way to do it, because it's all precisely time coded together. When it's done in post from two sources that were recorded separately and not time coded, sometimes the synch can be a little off I think, even from two all digital sources.