HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

As I understand it, the only settings that are truly strict – meaning violating them makes map generation impossible – are size and maximum number of players. And even some (or perhaps all) templates can be generated in two (or more) size variations, while you just need to not exceed the maximum number of players; you can reduce it, and the map will still be created.
The remaining settings (difficulty, water, monster strength, etc.) are up to your preference or according to tournament regulations.
You can see the settings with which certain templates are usually played here:
/topic/6313/
/topic/7430/
http://www.heroesportal.net/tavern/?id=285954
http://www.heroesportal.net/land/rules.php?type=on3