Apple Logic Express 9 - Manual (Page 287)

Send effects are also called bus effects, bus send or bus returns, auxiliary send or auxiliary
returns, or simply send or returns.
In Logic Express, the send effect is placed in an Insert slot of an aux channel strip. The
signals of the individual channel strips that you want to process are sent to this aux
channel strip, using a bus. The amount of signal is controlled by a Send knob on each
channel strip. The audio is processed by the effect inserted in the aux channel strip, and
is mixed with the stereo output.
The primary advantage of this approach (over inserting effects on tracks) is efficiency.
This method allows multiple channel strips to be processed by one inserted effect, which
saves vast amounts of processing power (and your time) when compared to the alternative
of inserting the same effect directly into multiple channel strips.
Another bonus is that you can quickly switch between wet and dry versions of all channel
strips that are sent to an aux channel strip, by simply bypassing the effect on the aux.
Similarly, you can completely change the effects configuration for multiple sent channel
strips by choosing different effects for the aux channel strip.
287 Chapter 10 Working with Instruments and Effects