Template T specifies the base grid class this class should derive from. It could be either a CGXGridWnd or CGXGridView derivative.
The CGXMRTGrid implements the logic involved in providing a look and feel of mulitple-row types, meaning each row with a different number of columns with each cell having a unique column width.
Here is an example grid view class that derives from this template:
class CMRTView : public CGXMRTGrid<CGXGridView>
CGXHierarchyGrid