A matrix which has previously been compressed from zones into, say, groups may be transformed back into a zonal matrix but with very specific rules which differ from the rules applied for compression or aggregation (10.4.4).
Thus under compression zonal cells are added to create group cells; by contrast under expansion all zone-to-zone cells take the same corresponding group-to-group cell value, i.e. group-to-group values are replicated. Mathematically:
This could logically be applied to a matrix of, say, OD costs or factors but not to a trip matrix where the group-to-group trips would need to be factored down when disaggregated to a zonal level.
Note that MXG2Z sets the intra-zonal trips to zero. This may have implications in later calculations if your original zonal matrix had values in the intra-zonals as MXZ2G would have included these in any intra-sector totals.