If you download the sample file, it shows three "sets" being used: Hitting, Fielding and Positions. Basically it allows you to organize your input variables.
For example, you may want to assign positions before the game is played. After it is played you would then enter in hitting and fielding stats. Sets allow you to quickly toggle between the different input groups instead of trying to jam in all the inputs onto one screen.
Link to sample file: http://statbook.pachysoft.com/download.html