There are three hint level available.
Level 0
No hints. All possibilities are available.
Level 1
Only numbers absent in row, column and square are displayed to click on them.
Level 2
Program performs more complex board analysis and eliminates more numbers than in level 1.
Highlight lonely numbers
This option is available in levels 1 and 2.
Error
Sometimes, in level 1 or 2, you will see empty, pink squares. This means that there is no number to put in. You should press Back and try another move.