This is a border selection operator, called Border Deselect Outer. It deselects editmesh geometry, that is outside of the border selection you make with this modal operator (like regular border select). Only previously selected geometry is considered, it won’t border select geometry that wasn’t selected before. This is useful to create a selection intersection from previous selection.
How to use:
View3D > EditMode > Select
Make a selection, then run this operator to border-select the desired selection intersection