What is the difference between a PZM microphone and a PCC microphone?

While both microphones are considered boundary or surface-mounted microphones, they are quite different in application and use. The Pressure Zone Microphone (PZM) has an omnidirectional capsule that "looks down" at the boundary, while the Phase Coherent Cardioid (PCC) microphone has a supercardioid capsule that "looks across" the boundary.

The PZM has a hemispherical pickup pattern when used on a boundary like a floor, wall, ceiling or tabletop. Another way to look at that is to say that it is omnidirectional above the boundary plane. This makes sense when you consider that it generally can't pick up sound from behind the boundary. The PCC microphone is a directional (half-supercardioid) boundary microphone that rejects sound from the side and rear. This directional pattern adds gain-before-feedback.