The concept of Most Favoured Nation status is older than WTO, dating back to at least GATT. But under the WTO agreements, it is treated as an obligation of members, with exemptions. For example, in the WTO General Agreement on Trade in Services.
Article II: Most-Favoured-Nation Treatment
With respect to any measure covered by this Agreement, each Member shall accord immediately and unconditionally to services and service suppliers of any other Member treatment no less favourable than that it accords to like services and service suppliers of any other country.
A Member may maintain a measure inconsistent with paragraph 1 provided that such a measure is listed in, and meets the conditions of, the Annex on Article II Exemptions.
The provisions of this Agreement shall not be so construed as to prevent any Member from conferring or according advantages to adjacent countries in order to facilitate exchanges limited to contiguous frontier zones of services that are both locally produced and consumed.
The above-mentioned annex for exemptions is where matters get tricky, and beyond my ability to explain. It seems possible that each member country can propose exemptions applicable itself. But that they are subject to review by the Council for Trade in Services. (There are probably different councils for different agreements.)
Given the two choices in the question, (fine-grained and detailed vs a set of general principles) it seems somewhere in the middle, but leaning toward general principals.
Exceptions to WTO Rules: General Exceptions, Security Exceptions, Regional Trade Agreements (RTAs), Balance‐of-Payments (BOPs) & Waivers seems to try to explain the guidelines. I'll pass on even reading it.