To swap to the mesh of cartesian type is necessary to select in


Meshing->General: Cartesian

Changing the kind of mesh type some other branch of Meshing could change: once applied the change it will appear a new 'cartesian' branch, and other options that does not have sense in this case will disappear.

A cartesian mesh is special kind of meshes, used for example by Finite Differences solvers, it is a grid with all edges aligned to global X,Y,Z axes.