P-carriage typically comes with ribbers, standard-, fine- and bulky gauge. But it will work with the main knitting machine bed. P-carriage only moves/pushes the needles. It does not manipulate any stitches (so it does not knit).
It assists the machine knitter in pushing the needles into various working positions, like from B to C, etc., depending on the starting positions of the needles.
It is useful to move needles for example to D position (out of work) or to C position (to then later transfer stitches from the ribber to the main bed).
It is also used for pile knitting or for drop-stitch lace (“drive” lace).
It is also useful to move needles to a more favorable position (for example after manipulating stitches, i.e., for cables) and then knit with a machine.
The look of the P-carriage for bulky and standard machines is a bit different.
P-carriage for standard machines has a little knob.
P-carriage for bulky machines is more square.
There is also a P-carriage for standard machines that is also equipped with a removable P-presser (a metal spring-loaded addition).
My Brother has a similar carriage, called D-slider.