The Boolean Cleanup tool is used to merge vertices at the intersection of boolean operations.
It does that by sorting vertices according to the edgeson each side.
Often with boolean operations, you want to maintain the vertices from one of the operators, and merge the other verts to those.
Select a loop of edges or vertices. In the context of booleans, this will be a closed loop, a cyclic selection, but the tool will also work on non-cyclic selections.
edge or vertex loop, cyclic or non-cyclic