I must admit I'm a bit confused with all the different names given to "sub-mechanics".
Aspect, skill and stunt, that's from Fate Core, so I dig it.
But tag, traits, attributes, aptitudes, fields... err, I'm getting confused now.
Maybe the guide assumes the reader is familiar with EP rules?
I must admit I never was really interested in the rules of EP. I've read the book but I skipped most of the "crunch". It's the setting that made me buy the book.
Personnaly, I don't really like all the "sub-sustems" introduced to the very clean Fate Core system. I feel it overcomplexify Fate Core rules (that I like because everything is very clean).
I feel like Skill Fields would be better handled with Stunts, as describe in the Specializations section. It would be totaly on par with the Fate ruleset and will remove the sub-rules on how fields can be apply to certain actions