The aim of solving a sudoku is to use logic and not to guess or try.

Guessing and trying is required when the sudoku has multiple solutions or the skills to solve it are not high enough.

It is a common agreement that sudokus with multiple solutions are invalid. Luckily such sudokus are usually not published or just published by mistake.

If the skills of a human or program is not high enough then guessing is a not very satisfying way to get the solution.

Unique Rectangle

The fact that a sudoku can have just one solution can be used to eliminate pencil marks.

In all four corners have the pencil mark 4 and 2. To not run into multiple solutions the pencil marks 2 and 4 can be erased in the lower right cell so just the pencil marks 3 and 6 remain.

The above sudoku has a second unique rectangle, the upper left cell is a 9:


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