Look at row A of the puzzle. Cell A5 is highlighted in blue, and candidate 7 is further highlighted in cyan. If you examine the rest of the
row, you will see that candidate 7 does not appear anywhere else in the row. Therefore, 7 must be the solution for the cell A5. Numerous
other example can be found in the puzzle: eg. Cell C7 contains the only appearance of candidate 4 in row C. Interestingly, it is also the
only appearance of candidate 4 in column 7 and in box 3.
Puzzles which are created at difficulty level 2 can be solved using just this technique, plus the Single Candidate technique.