Volume pedals are hooked up between an AUDIO line. They use a CVA to modulate the volume.
They cannot be recorded in a sequence.

Expression pedals are used when you are recording. They send Controller number 7 to the sequencer and "stairstep" the volume in 128 steps. A really good one is pretty transparent. Many keyboards have an input for an expression pedal which is connected to the MIDI volume controller.

Volume pedals have to be connected between the audio output and the amplifier input. they have 2 jacks. IN/Out

Now both can be considered a volume or expression pedal however it's important to know what they are doing.

You can assign an expression pedal to the volume parameter.

The pedal you are describing seems to use both a VCA and the ability to control a MIDI signal.....

[This message has been edited by Kingfrog (edited 04-28-2009).]