Think of it this way, a fillet weld goes on the joint, a groove weld goes in the joint.
Do not confuse the type of joint with the type of weld. Example; a T-joint can be joined using a fillet weld on one side or a fillet weld on both sides of the butting member. The T-joint could also be joined with a complete joint penetration groove weld, a partial joint penetration groove weld or even CJP or PJP groove welds on both sides of the butting member. To complicate matters, the groove weld can also be supplemented with a reinforcing fillet weld.
This is a case where the use of standard terminology prevents miscommunication.
In the world of AWS, a butt joint is one of the five common joint types; T-joint, Corner Joint, Lap Joint, Butt Joint, or Edge Joint. A T-joint or corner joint joined with a CJP or PJP groove weld is not a butt joint.
There may be and most like are differences between AWS and ISO terminology. I find it confusing to say any joint that is joined by a groove weld is a butt joint. I prefer the AWS terminology where the butt joint (both members are parallel on a common plane) is joined with either a CJP or PJP groove weld. You can't join a butt joint using a fillet weld.
Best regards - Al