The term "Matrix" is not one you will find with an explicit entry in the BDL documentation. It is the internal name given when elements are repeated in a container that is not intended for repetition i.e. not in a TABLE, TREE, or SCROLLGRID. You may see it inside a .42f, inspecting the AUI Tree, inside GUI logs etc.
If you do encounter Matrix, chances are it indicates that you have a repeating element when you don’t mean to, or you are using a GRID container when you perhaps mean to use TABLE, TREE, or SCROLLGRID.
Read more at
https://4js.com/ask-reuben/ig-138/