Digits confined to as many cells lock those cells. medium
A group of digits fits only in the tinted cells of the unit, so those cells hold nothing else. Erase 5, 6, 7 and 8 from them.
If a group of N digits fits in only N cells of a unit, those cells belong to that group, and every other pencil mark in them can be erased. The mirror image of a naked subset, and easier to miss.