Think of #iid as a filter on the wider porn caption caption archive: same submission pool, but slimmed down to the exact register you searched for.
The page pulls in iid-tagged captions, plus caption GIFs where iid is a secondary tag, plus roleplay posts whose title text overlaps with the iid keyword — three sources into one feed.
Because #iid is tag-locked, the recommendations under each post also stay in the same lane, which means fewer 'that wasn't what I wanted' clicks and more time on the actual fantasy.
Every visit reshuffles the sort weights slightly, so the same #iid archive rewards repeat trips — the top of the grid tomorrow won't be identical to today.