The TX sends out a carrier frequency and the signal basically shifts 5 KHz either above or below that carrier during operation for North American systems. Some manufacturers chose to shift above and others chose to shift below. The RX will only recognize the correct shift it was designed for. PCM adds another ball of wax to the puzzle, more coding of the signal.
Basically any MFG FM TX that SHIFTS in the same direction as another MFG FM RX should work together. This can not be said for PCM.

There are a few RX out there that can be used with either shift, either automatically or through programming. There are also a couple of TX that can be set to transmit either shift.