My opinion is that it is not always a T-Y-K connection, even though D1.1 is not very clear on the subject. In reviewing D1.1, I noted the following definitions:
tubular connection. A connection in the portion of a
structure that contains two or more intersecting members,
at least one of which is a tubular member.
tubular joint. A joint in the interface created by a tubular
member intersecting another member (which may
or may not be tubular).
In Table 4.10, a welder qualified on a plate test may also weld PJP and fillet welds in either pipe or box tubes in T-, Y- or K-connections. So if you are fillet welding slotted tubular braces, fillet welding smaller box sections to larger box sections, welding size-on-size box sections with a combination of fillet and PJP flare bevel groove welds, etc. a 6GR welder qualification is not required.