Short phrase meaning "Area of study, activity, or interest where a particular technology or method is used." [duplicate]

I feel like I know a phrase like [...] of [...] that is often used on the tip of my tongue, but can't spit it out.


  • Field of expertise
  • Field of study
  • Area of expertise

These are all interchangeable.

A similar question, albeit asking for a single word has been asked here. Answers included the words "discipline", "subject", "specialty" and, most interesting, the borrow word "métier".