Type 2 Unique RectangleA Type 2 Unique Rectangle occurs when two cells on one side of the rectangle have the same additional candidate. To prevent a non-unique rectangle, one of these cells must get this digit. So from the other cells that are simultaneously visible from these two cells we can eliminate this digit.
In the example below, the cells R1C9 and R3C9 have an additional candidate, the digit 2. To prevent the occurrence of a non-unique rectangle, the digit 2 must be placed in one of these cells. Thus, the digit 2 can be eliminated from the cells R3C8 and R8C9. After this elimination, the digit 5 can be placed in the R8C9 cell.