This is a notation that my professor uses, that I haven't been able to find anywhere else. It has to do with representative matrices for a certain transformation from one basis to the other, I believe, but I'm still not sure as to what it's supposed to represent. Does anyone know? Thanks.
I should have put this down before, but here is what my notes say:
Let $f: E_b \to F_{b'}$ and $B = \{e_1, e_2, \dots, e_n\}$, then $ \operatorname{Mat}_{b'}^b f = [f(e_1) \; f(e_2) \; \cdots \; f(e_n)] \, B^{\,'}$