Ding123
ASME VIII Div 1 UG-116 (e) gives you all you need to know about RT 1, 2, 3, 4.
It will also refer you back to UW-11, UW-51 for more info.
In a nut shell RT 1 is "full" RT since "100% RT" does not really exist.
RT 2 tru 4 are various spot RT cases. You need to read those.
What meteng refers to are joint categories and these are related to the extent of RT and joint configuration to give you joint efficiencies.
I hope it helps.