the site "board game geek" (a board gaming resource site) describes set collection mechanic as thus: The value of items is dependent on being part of a set; for example, scoring according to groups of a certain quantity or variety. https://boardgamegeek.com/boardgamemechanic/2004/set-collection

from what I can gather, set collection is a term used in board game design, which involves collecting certain matching elements called "sets".