[NTG-context] Can someone explain the \startuseMPgraphic interface to me?

In details.pdf, I encounter \startuseMPgraphic{mpos:par:columnset:shade} numeric h ; for i=1 upto nofmultipars : h := bbheight(p) ; if multikind[i] = "single" : fill multipars[i] topenlarged -.5h When I encounter that, I wonder: - where do nofmultipars, p, multikind, and multpars come from? - what does mpos:par:columnset:shade signify? Just a name? Do the ’:’ have meaning? G.