We ran into a fun one about Mist Horn:
POWERS : Display next to a location.
While displayed: When a character at this location suffers damage, reduce it by 2, or 1d4+1 if proficient.
TRAITS: MAGIC & INSTRUMENT
Question is : whose proficiency are we talking about? The one of the character at the location suffering damage or the one of whoever displayed the card next to the location?
Say LEM (proficient with INSTRUMENT) displays the Horn and VALEROS at the location (not proficient in MAGIC or INSTRUMENT) suffers damage. What is the reduction?
And if VALEROS displays the Horn and LEM suffers the damage?
I guess I know the answer but the wording isn't 100% clear.
The character playing the horn must be proficient.
To my knowledge, every extant instance of (an assumed) "you" (as in: "If (you are) proficient...")on a *played* card always refers only to the character who played it. (As opposed to, say, a barrier like Abyssal Shortcut, that makes you display it next to a location - at which point the "you" on it refers to each player at that location. Note that they FAQed the "you" on AS out for clarity. That seems to confirm that if they wanted an effect opposite to what skizzers said - they'd rather use "If a(ny) local character if proficient..." etc.. There's no specific written rule to codify that, that I can think of, if that's what you're asking for, though.)
That was my understanding too, and the way we play it, but I must admit, RAW, it is not totally obvious.