Essentially, a PIP card is a plugin module at the back of the amp that interfaces with the amp's main inputs. Crown has (or had) a lot of different cards that performed functions on the amp's input from simply interfacing it with either XLR or RCA, to active filters/crossovers, to remote attenuation that would be used in a PA system control/sound board.
The first comtech crown I owned had some fancy PIP card with balanced XLR's in the back and a digital attenuator/control bus. I got in touch with a contact at Crown at the time, and purchased a simple "straight through" XLR input card, and a pair of XLR to RCA cords.
The one I have now has a terminal block and has an isolation transformer, a stepped precision attenuator, a defeatable high pass filter, and something that they call a "Constant Directivity Horn" EQ circuit which introduces an EQ curve with a 3db rise at 3.2khz and peaking at 12.5db at 24khz. Something to do with specialized speakers??
I'm thinking about doing a mod to it and making it completely passive and "straight line". The one caveat I suppose is bypassing the isolation transformer, as it's purpose is to minimize noise and ground loops.