After reducing the block with possibilities, every cell in the block will have a set of possible values. In that, if a particular possible value is present only in a single cell. Then that value is the unique possible value for the cell in that block and the cell is assigned with that value.
Since the value 5 is present only in the single cell,it is the unique possible value for that cell in the block. Therefore, that cell is assigned with that unique value.
Figure: A Rectangular-Block from 9x9 sudoku.