You already know the Mirror Feature command from feature-based modeling (chapter 4.2.5,"Enumerative Features - Pattern Feature"). It is very similar to the Mirror Face
function explained in this section.
In Synchronous Modeling, however, you should note the difference between a feature and a face. A part of the body may look like a feature (e. g. a cylinder) to you, but for the program NX it is only a face with a certain geometry, because the program does not know the creation history. These faces are called "Face Chains".
Therefore, the Mirror Face command is especially relevant for Synchronous Modeling.
It mirrors a face or feature at a (help) plane selected by you. To do this, first click on the surface to be mirrored and then click on the layer on which you want to mirror.