The “unique” strategy looks for candidates that are unique within a unit. This happens when all other candidates for the same number are removed from the unit (see Figure 4-1).
Figure 4-1. “Unique” within a row
If Solver, while applying another strategy, happens to remove the candidates for 5 from cells 7 and 8 of Figure 4-1, the 5 in cell 4 becomes unique within the row and, as a result, the candidates for 4 and 8 in cell 4 can be removed.
To be consistent, the name of this strategy should have been hidden single, ...