The units column 9 and box 6 intersect at the cells D9, E9 and F9. Of the candidates remaining in these three cells, 1 is the only one
which does not appear elsewhere in box 6. Therefore, the solution cell for 1 must be one of the blue highlighted cells. We can't tell which
one it will be, but we do know that none of the other cells in column 9 can have a solution of 1. In other words, the magenta highlighted
candidates can not be solutions and may be removed.

Unit Intersections may also occur between rows and boxes.