matrix~ is an array of signal connectors and mixers (adders). It can have any number of inlets and outlets. Signals entering at each inlet can be routed to one or more of the outlets, with a variable amount of gain. If an outlet is connected to more than one inlet, its output signal is the sum of the signals from the inlets.
A quick look at the matrix~ [reference http://www.cycling74.com/docs/max6/dynamic/c74_docs.html#matrix~] file should make it clear how matrix~ can be used within you msp patching environment.
Via the inspector or attrui object, a ramp time can be set to fade between inputs in a cross-fader type fashion.
(please list things that you believe to be errors or omissions from the existing refpage)