More than one override mesh regions: In some cases more than one override mesh is needed. Due to the meshing mechanism, from finer mesh to coarser mesh, it has gradual change. By default it has the graded factor 1.414. Therefore, for layered structures or when you add two cascaded override meshes, the actual mesh will not behave as expected. For example, one layer has a thickness of 30nm, and the other has 70nm. You add two override regions: one has 3nm mesh size and the other has 7nm to expect integer number of meshes for each layer. But the result will have some graded meshes from the finer mesh 3nm to 7nm.

I would suggest to test the meshing if you have complicated structure and more than one override regions. Please note that the override region will have one mesh buffer on each side.

You can set the override to have 1D,2D or 3D specified mesh sizes.

The override region can also be attached with a specified object.

Please note that in most cases the override mesh may not be used to release the heavy memory burden. Otherwise the result accuracy can be poor, except that you know the reason, such as in this example: https://optics.ansys.com/hc/en-us/articles/360042357654-CMOS-Optimize-microlens-ROC