I have a six-dimensional matrix $X_{a,b,c,d,e,f}$ with indices $a$, $b$, $c$, $d$, $e$, and $f$. For convenience, I would like to substitute ${_{b,c,d,e,f}}$ part of the subscript with a single $g$ index in mathematical text, in order to be able to just write (and talk about) $X_{a,g}$ instead of the long $X_{a,b,c,d,e,f}$. How should I call the substitution? In computer science, we would just say that we have “flatten” the six-dimensional matrix into a two-dimensional matrix.

Note that $_g$ fully determines $_{b,c,d,e,f}$ and $_{b,c,d,e,f}$ fully determines $_g$.