You should ensure there is only one solution before attempting an analysis. (see checking)

The puzzles are classified according to the strategems a human solver would have to use to arrive at a solution.

easy
just sole candidate and sole possibility
medium
as above plus row and column fragments and ngroups and npossibles of two cells
hard
as above plus larger ngroups and npossibles
very hard
x-wing, swordfish, conjugate pair chains
impossible?
cannot be solved by the above means, nor by this program acting in 'human' mode