a return bus is a separate input (usually no frills) on a hardware mixer that you return a send.  It can be thought of as a line input, as opposed to a full channel w/ preamp, more sends, EQ, buses since it returns the send directly to the master for mixing in reverbs, etc.  A lot of hardware mixers have them.

In Sonar we don't really have them, since every send or bus can hook into whatever channel you like.  Still, every send (except headphones) will likely need to be returned to your system, so the terminology sticks.

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