Question

What does it mean to have two matrix variables written vertically inside parentheses?

In other words, if A and B are matrices, what does this mean (imagine there is one set of parentheses spanning both rows):

(A)
(B)

I'm trying to understand a least squares problem like this:

(A) * C = (D)
(B) (0)

I understand regular least squares problems with matrices, but the parenthesis notation is confusing me.

Thanks!