The PC button is one of the buttons that selects the input going into the projector.  The PC button does not turn on the local PC, this needs to be done using the on button on the front of the PC.

When the PC button is selected the video output from the local PC goes to the local ceiling mounted projector (or in a few cases a monitor screen),  Also, the audio output from the local PC goes to the room's wall mounted speakers (or in a few cases the local monitor speakers)

The button panel controls the AV switcher in the desk rack.  The PC input can be selected using the AV switcher if the local button panel is not working.