When using phases with Updated Mesh (UM), the Safety analysis will use the deformed mesh (with the updated node locations) as a starting point, while the node locations are not updated any more during the Safety analysis.
Deformations that you get from a Safety analysis are fictitious (see the related article on Safety analysis and displacements) and are only used to drive the process and to indicate the failure mechanism (displacement increments rather than total displacements).
If Updated Mesh would be used during a Safety analysis, the geometric changes would become unrealistic. For example, a Safety analysis with Updated Mesh performed on a slope would lead to the slope becoming flat in the end, with an infinite safety factor, which is not the desired outcome of such a calculation.
During a safety calculation, you will get additional displacements (in the deformation parameter u), but the node locations will not change. This can be of great importance when an embankment in the undeformed state is too steep, while in the deformed mesh it is stable.
Another example for which this can be important is an embankment that will sink partly below the water table due to its self-weight when using the Updated Water Pressures option (available in PLAXIS 2D). This will reduce the effective weight of the embankment structure on the subsoil.
Safety analysis and displacements
[Tips and Tricks]
Safety analysis and undrained behaviour
Safety calculation with a Target Value