A rule governing the role play of character skill vs player skill. From the Accelerant core rules:
"If there is a skill that specifically allows you to manipulate a prop or perform some action, you cannot attempt to perform that action or manipulate that prop unless you have that skill. You cannot pick up weapons and try to fight unless you have the skill to do so. You cannot manipulate traps unless you have the skill to do, although you can attempt to avoid them."
Even though a player may have the physical skill needed to perform an action, if it is not an ability selected during character creation, the character will be unable to do that action.
This largely refers to physical interactions - just because a player can physically do it does not mean they should, because their character may not be able to.
The "Always In Game" rule of Etiquette makes it difficult to restrict matters of player mental comprehension.
For example, if staff wrote a "secret code" but wrote it in Latin, and a player studied Latin out of game in school for 4 years, that player is highly likely to understand the code even though "Latin language" was not on the character's list of abilities.
Despite this, players may choose to act in good faith according to the role they are trying to play. In that same example, the player who understands latin, but plays an illiterate barbarian, may opt to simply refuse to look at the writing because their character has no interest in such things.