hard tier
Look at the pencil marks in the two highlighted cells, both in the same row. Digit 8 only appears as a candidate in these two cells within that row — and they happen to share a 3×3 box.
Since 8 has to go in one of these two cells, the rest of that box can't be 8 — so 8 can be crossed off the marked cell, even though it still has other candidates too.