AudioDesk, Digital Performer 2.2 and later have the ability to use a bus to internally route an audio signal from one place to another. The most common use of this would be to use a bus to set up an effect send, the same way you would use an aux send and return on a mixing board.

To do this, select "Add Aux track" from the Mini-Menu of the Tracks window. After selecting either a mono or stereo aux, set the input of the Aux track to a bus, or pair of buses if stereo. You will then want to open the Mixing board and show the sends by selecting them from the Mini-Menu of the Mixing board. There are four sends per track. The drop down list directly above a pair of sends is for the left send knob, and the list below is for the right knob.

Next, select the the appropriate bus for the send of the track that you wish to send to the Aux. Set up an effect on the Aux track, and adjust the send of the track to add the desired amount of effect. You are sending a signal to the bus, and returning the bus to an Aux track that contains an effect.