What is a valid PLN flight plan?

Basically, I'm trying to make an XML schema against which I'll try to check and validate a .PLN flight plan (FSX and P3D) using my validation tool and return true if it passes the validation successfully. I'm using FSX but I suppose the content of a P3D flight plan is based on the same XML architecture.
So, is there any comprehensive reference or manual that I can check for all the valid elements of such a flight plan instead of deducing them from the ones generated by third-party tools?
Apart from basic elements such as Simbase.Document, start location, cruising altitude, flight type (IFR/VFR), etc., I know that any .PLN may include navaids such as VORs and NDBs, Intersections, User waypoints, Airport, ICAO Region, ICAO ids, ATC Waypoint ids...
Anything else I forgot? What about TACANs, for example?



Resource contributor
I don't know of such a document. I would suggest allowing FS to create many such plans (including TACANS if available in FS) and examine them for the proper coding.