A matrix stacked by blocks (10.2.4) may be transformed by swapping blocks into levels. Thus the sub-matrices in Figure 10.1(b) would be moved into the positions in Figure 10.1(a).
Prior to version 11.5 this was treated as a form of matrix transposition
This is a useful “trick” since at the moment MX is better equipped to display information relating to sub-matrices by level rather than by block; e.g. printing cell values or row/column totals interleaved.