PB's in the F/A-18C designate the buttons around the AMPCD and DDI's (20 each). The screens show the current options of the buttons next to the PB's. When pressed, the screen either switches to the specified page, or the activated option is framed to indicate that it is selected or active. If there is nothing next to the PB, it has no function at the moment.
In our courses we name the buttons after their function. In other sources they are numbered consecutively (from the bottom left corner clockwise e.g: PB 19 is in the bottom row of the 2nd from the left).