In the example you see the trivial placement of the number 9 in row 2 and column 9, because all other candidates in the box are eliminated due to the number 9 in row 3 and the number 9 column 8 (see the red lines). In the same way the number 9 can now be placed in row 8 (and in column 7).
We now used only the first part of the definition of a single candidate. The second part of the definition can be used, when the puzzle is almost finished, to find single candidates in a unit, where only a few unsolved cells (or just one) are left in a unit. Then it is easy to check what possible values can be entered for these cells. But be aware that some sudoku puzzles are designed such that you can only start solving them with the second part of the definition of a single candidate.