It can refer to black belts, but it doesn't have to. Usually, when referring to non-black belt students, we tend to say "underbelts". When we want to include black belt and those students close to black belt, we usually use the phrase "senior students".
A belt worn by a senior citizen, who happens to be taking Taekwondo.
Yeah its not something i use much anymore, i prefer to use the Black belts and then Advance, Intermediate and beginners.
This is a comparative term, with no absolute meaning. It refers to the more experienced and higher ranking members of any specific group.
