PC board vs add-in

PC board and add-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
PC board	Yes	No	No	No
add-in	Yes	No	No	No

As nouns, add-in is a hypernym of PC board; that is, add-in is a word with a broader meaning than PC board:

PC board: a removable circuit board for a personal computer; fits into a slot in the mother board
add-in: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities

Other hypernyms of PC board include board, card, circuit board, circuit card, plug-in.

PC board (noun)	add-in (noun)
a removable circuit board for a personal computer; fits into a slot in the mother board	a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities