A sudoku is only a sudoku if it has exactly one solution. A grid with two valid answers is a puzzle that punishes a solver for reasoning correctly, and it is the single fastest way to earn bad reviews on a puzzle book.
This generator produces a complete valid grid, removes clues while checking that a unique solution remains, and returns the puzzle together with its answer. Difficulty is a function of how many clues are left and which cells they occupy, not simply of how many numbers are missing.
That distinction matters when you are building a book. Two puzzles with the same clue count can differ a lot in how hard they feel, because what makes a sudoku hard is the solving techniques it forces rather than the size of the gaps.
Yes. Clues are removed only while a unique solution is preserved, which is what separates a valid sudoku from a grid that merely looks like one.
The solving techniques it forces. An easy puzzle can be finished by scanning rows and columns; a hard one requires reasoning about candidate positions. Clue count correlates with difficulty but does not define it.
Fewer clues generally means harder, but placement matters as much as count. The generator handles this by validating uniqueness rather than by hitting a target number.