The English wording seems unclear on whether you gain the MR *against* spells of that path cast on you, or if it makes spells that you cast harder to resist.

"The character has a special bonus of 20 to his MA and to his MR in the path he chooses" implies that it's "his MR," or the character's own resistance.

However, the next sentence states, " When he uses spells of the opposed path, he has a penalty of -20 to his MA and to his MR" (emphasis mine). If the character is the one using the spell, he doesn't need to use his own MR to resist it, so this seems to indicate that the bonus/penalty applies to the roll to resist the character's spells.

Is this any clearer in the Spanish (or French) rulebook?

Soyou found some "unclean wording" uh? Welcome to the club

Anyway, the Spanish version of the Revised says exactly the same. To the point that I had the same question you had.

I've seen 2 interpretation:

1) The caster's MA suffers the penalty, and the same penalty applies to _his own spells_ (not to the caster).

So if the spell says "RM 120", with the penalty would make it "RM 100", and so easier to resist for an opponent.

2) If you specialize in Fire, you get the -20 MA when casting Water's spells, and if someone else cast a Water spell on you, you suffer the -20 RM to the resist check (if a check is required).

The difference is if YOU suffer from the -20 or YOUR SPELL suffer from the -20. The manual says "his RM" (from Spanish version), so I'd stick with "you become an easy target for the opposite school magic, so you get a -20 on your RM check against that school", the 2nd option.

I didn't bother to get over this cause I wasn't interested enough in that particular Advantage.

The correct interpretation is the 2°.

It's like Dark Form, it gives to YOU +30 against effects based on Darkness, because you are made of pure dark energy.

With Elemental Compatibility, if you choose Fire, you gain +20 to YOUR MR against Fire, but -20 against Water.