What does the "Not Solved" grade mean?
A. This grade means that Sudoku is not able to solve the puzzle without resorting to trial-and-error.
The "not solved" grade was added with version 3.6, released November 2010, in response to the increasing number of requests for even harder puzzles.
If you enable trial-and-error as a solving technique, Sudoku will still be able to solve puzzles of this grade, but there will be at least one number whose placement cannot be justified by a logical deduction - Sudoku made a guess in other words.