I've seen the term Faerie Correspondence tossed around a couple of times - however I've never seen it in any of the Ars Magica 5th rulebooks, and my collective is pretty comprehensive...
Am I missing something, or is this from a previous edition?
I know people make a lot of references to faerie Sympathy and sympathy traits, or possibly use of charms. If you're ever trying to get an ability total really high, having an appropriate Sympathy trait makes a big difference. Charms are referenced when someone's trying to get an obscene penetration total. I think people may be referring to one of those two.