I'm looking at the tag descriptions for INDO_IDENT and REFN and somewhat sound the same
REFN REFERENCE A description or number used to identify an item for filing, storage, or other reference purposes
IDNO IDENT_NUMBER A number assigned to identify a person within some significant external system.
I want to track ID numbers relating to a couple of sources I'm using. The tags would be set to ADMIN only. Is REFN considered only relating to the INDI number of an individual GEDCOM record?
the INDI number in PGV (or webtrees), properly called an XREF, is not any sort of stable number. They are not unique, they don't track to any system and they don't relate to other methods.
You answered your own question when you posted the definitions. REFN is something you create that can be used to indentify an item for filing, storage or other reference purposes.
IDNO is ONLY for persons and is usually designed to coordinate with a significant external system.
You would/should use only REFN. They should be UNIQUE and can be applied to more than just people.
In my cleanup I'm catching some of the inconsistencies I've made using IDNO, REFN and AFN. this number I'm using will not be unique per person but a group, usually family group. The source uses numbering 1, 1.1, 18.104.22.168 etc but so far I only used the top number.
For the first source, each chapter numbered the head person and I used that. It's on my long to-do list when I continue that line to make the numbers more unique.
For the second source, each head had a number and each child a subset except it didn't continue down to further generations and when I tried to map it, took an extraordinarily amount of time to do manually. There's 2-3,000 names. So kept with the numbers used in the source which in turn relate back to the book. Author has 5 family groups (A-1, B-1, M-1, N-1, NM-1) which in turn may/may not extend to next level of A-1A depending on how many children/dependents he tracked.