If the zone structure has been transformed using any of the above options then the original matrix/matrices are effectively “lost” having been replaced by the new structure. The reboot command allows the original command line to be re-processed and the original matrix/matrices to be restored (at the cost of losing the “new” restructured matrix data).
The idea is that if you input a zonal matrix and then compress it to a group matrix (10.4.4) in order to “view” the matrix in terms of group-to-group you may return to the original zone-to-zone version. This therefore gets around a “feature” of MX whereby you cannot store a zonal matrix and a group matrix in internal memory at the same time; now you may swap between them.
(By contrast sector-to-sector versions of the basic matrix may be stored at the same time as the full version and both may be viewed within the same program; however there are severe restrictions on the size of sectors.)