matrices – Substitution of a set of variables into a single variable

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 $$_g$$.