add-in vs PC board

add-in and PC board both are nouns.

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

As nouns, PC board is a hyponym of add-in; that is, PC board is a word with a more specific, narrower meaning than add-in:

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

Other hyponyms of add-in include CPU board, mother board.

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