Dear Steve,
there is a lot of information in the help file of the Areabook, you can press F1 after you have opened the areabook or just go to: C:\Program Files\Autodesk\Roombook Areabook Buildingbook for Revit 2020\Areabook\Help
In there you should find all the information, also dedicated schedule templates that you can import.
I don't exactly know the English abbreviations NRA, GFA and LA, it should also be in the help file, but I guess it's the net room area, gross floor area and maybe lettable area?? Depending on rooms, lettable or gross area they are using the dedicated wall lines.
You can also have a look at this video, it's some years ago but explains very detailed the whole extension: https://www.youtube.com/watch?v=wPdZdge37hA
Hope this helps your understanding of the areabook a lot more now 
Please mark the answer as solution if you are satisfied with it. Otherwise just reply so we can find additional solutions for you.
Best regards, Jochen
Jochen Tanger
Sr. Technical Specialist AEC
BIM Blog