Governing moment reported by the calculation sheet is the maximum out of the moment values obtained out of all the grid points. These grid points or design points are the ones that one defines through the Moment Envelope generation page within the Mat slab analysis/design options. The moment value depends on the grid divisions that one defines by setting the division along longitudinal and transverse axis. The Mx and My reported in the Plate Stresses table on the other hand are the values of moments calculated at the center of the plates. So there can actually be differences in between the two moments if the locations of the grid points and the centers of the plates do not coincide.

One more thing which is worth noting is that the My and Mx that are reported in the tables, are in terms of the local coordinate system of the elements and these moments are local moments. These local directions may not match with the longitudinal or transverse directions in which the design moments are calculated. The longitudinal and transverse directions by default correspond to the global X and Z but can also be specified by the user through longitudinal axis setup within Moment envelope generation. When calculating the reinforcements for the zones ( governing moments) , the moments are resolved along the longitudinal and transverse directions and reinforcements are calculated along those directions.