Oh, another example of the distinction between secret information and secret rules: the stress and fallout mechanics of Spire. In this system, when you take an action you will often take stress. Stress is assigned to one of five tracks. By itself, stress does nothing; a character with 10 stress is equally likely to succeed at an action as a character with no stress, other things equal. However, every time you take stress the GM rolls a die. If it's less than your total stress (across all tracks) you take fallout, reducing your stress by a certain amount but taking mechanical penalties as well, up to and including death. The rules for stress are not secret; there are player-facing mechanics like class abilities that directly interact with stress tracks, and you can't really decide what abilities to take unless you know how stress works. However, once you are playing, all the rolls relating to stress are made by the GM and the numeric results are secret. The GM is expected to narrate the effects of increasing stress, and the players need to evaluate how bad things are likely to be based on that narration. This is great and works very well in play, in my experience, and it's a clear example of a setup where secret information makes the game more fun, but where the underlying rules are public information.