What does $_A\backslash B$ mean?

Question

I came across the following notation in this document

$_{GL(k)}\backslash \text{Mat}^*(k,n)$

What does $_A\backslash B$ mean, where $A$ and $B$ are suitable mathematical objects?

Answer 1

When a group $G$ has a right-action on a set $E$, $G\backslash E$ denotes the set of orbits of this action.