me again newbie,
the black semi-circular plastic piece is for routing round a raised lip to cut the outer side, it should be mounted to the baseplate with the baseplate on a flat surface (like a table top), with the cutter sticking out over the edge banding (or raised lip) and brought in to thin the banding piece.
at least that is what I can make out of the instruction book.
sorry for the delay in answering, but the weather makes my workshop unusable,so couldn't get to the instructions.
if I am wrong about this, I hope someone will step in with the corrections,
personal I can't see much need for this piece.
all the best
pip