The RT probably stands for Radiographic Testing (RT). Not sure about the suffix numbers but I would guess that the 1,2 and 3 could possibly stand for a level I examiner, 2 would be a Level II examiner and 3 would be a level III examiner. Where did you see these specific designations?
Ok. In ASME Section VIII, Div 1 Table UW-12 contains 8 weld joint types for RT. This could be what you are referring too;
RT Type 1; double welded butt joint
Type 2; single welded butt joint with a backing strip
Type 3; single welded butt joint with no backing strip
Type 4; double fillet welded lap joint
Type 5; single full fillet welded lap joint w/plug weld
Type 6; " " " wo/plug weld
Type 7; corner joints, full, partial or fillet weld
Type 8; angle joints
Please refer to this table for a complete description.
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.
