One can use the Faerie Magic ability when practicing magic associated with the Faerie realm. Is that referring to the Faerie Wizardry chapter in RoP:F only? Or, could a magus use it when researching spells associated with faeries, such as Faerie Chains of the Familiar Slave (HoH:MC, pg 90)?
The context of that particular phrase is that it is used in the place of Magic Theory for certain lab activities (that's the only place Magic Theory is used in the practice of magic). I would definitely say it applies to spells that involve faeries as Targets or recipients of spells and also any spell that uses the R/D/T parameters that Faerie Magic grants access to.
Most Faerie Magic characters will generally have their Magic Theory score close to their Faerie Magic score to maintain flexibility in learning non Faerie Magic spells, and also to take maximum advantage of using all Faerie Vis, but I can see cases, such as someone with Affinity and/or Puissant Faerie Magic having a score well in excess of Magic Theory and having a different lab total for faerie focused spells...
I'm pretty sure that Realms of Power:Faerie was published after Houses of Hermes: Mystery Cults (quick scan of the book is 2011 and 2006, respectively). For that reason, I seriously doubt that it applies only to the Faerie Wizardry chapter in RoP:F, and has a bit broader application.
Is this what you're trying to understand?
Yup. I was interpreting the rule the same as you, anything involving Faeries can use the ability.
That's what I intended when I wrote it, anything faerie. Also doing lab work in a faerie aura, or Hermetic projects that use faerie vis.