There are four puzzle library levels corresponding to the four levels of strategies described in the Fundamentals, where text and simple graphics are used to show how these strategies follow logically from the basic Sudoku rules.
Within each library level, a Grader assigns stars indicating difficulty by determining which strategies at that level are needed to solve a puzzle, and how often they are needed.
For instance, all puzzles in the Basic library can be solved with minimal visual aids. But if you select a 3-star puzzle from that library, you will find that it will take you awhile to solve it.
An option is provided that will cause the display of the abbreviated names of which strategies are needed to solve a puzzle, whether it is loaded from a library, created or imported. See the end of the topic "Creating Puzzles" in the Fundamentals to interpret these abbreviations.
A fourth star is used at the Extreme level to indicate the puzzle could not be solved using the current Extreme strategies listed in the Fundamentals. Try one of these, and if you can't solve it, you can export the puzzle to a web-based solver where the solution steps are shown.
In addition to the library puzzles, you can create your own puzzles, using both manual and randomly generated input. Random clues are evenly distributed in rows, columns and blocks, unless forced into a pattern that you have defined.
Besides providing information on the strategies needed to solve a puzzle, the Grader also provides statistics that are useful in judging the quality of a puzzle.
You can create a random puzzle for one of the 13 puzzle levels. The Random action then creates puzzles in the background, displaying in the foreground that puzzle which currently is the best match and has the best quality based on grader data.
Each library level has a "toolkit" of visual aids appropriate for that level; from nearly none at the Basic level culminating in a "draw mode" at the Extreme level, which lets you draw a chain of implication links between possible cell solutions.
The last screenshot of an Extreme puzzle shows all the visual aids. This 1-star Extreme puzzle has a basic chain with possible endpoints that are strongly linked: if one endpoint is not a solution, then the other is a solution. Possible 5 in row 6 column 4 that "sees" both endpoints can therefore be eliminated, and the puzzle is shortly solved as a result.