Thanks for these answers! And one more, if possible. "HD is also added to tests rolled by monsters if they are particularly suited for them" - can you bring some examples of such tests?
Sure! As Monsters don't have Ability Scores, they can use their HD as a bonus to any tests if the Referee deems it fitting. For example, an ogre climbing up a tower wall, a dragon sniffing out a thief, or a goblin hiding.