Parts "cut-off" with a plane
 

Hi,
I have a domain containing several parts and regions.
I would like to be able to "cut" all the parts that are above a plane parallel to the (XY) plane, so as to create a sort of 2.5D region of very small Z height.
How could I do?

Thanks

The ultimate goal would be to thicken the polyhedral mesh that I already have in 3D, to do this I want to decrease the height z, without scaling the model, but just cutting it.

why dont you use the 2d mesher as explained in the help here Pre-Processing > Meshing > Two Dimensional and Axisymmetric Meshes > Creating a Two-Dimensional Mesh

Because I would like to keep the same relationships, volumes and control surfaces that I have in the current volume mesh operation. I tried with the "convert to 2D" command but I don't know how to go forward because I have 2D regions and a volumetric mesh operation on it, I don't know if it will work.

if you follow the help carefully when using the older ‘convert to 2d’ method then it works very well and retains all the surface and volume meshing controls you applied to the 3d mesh - so ensure your source 2d surface at z=0 on the x-y plane is very flat. you have the option to retain the 3d mesh and this can be rendered inactive during solving by removing its physics.
but the most flexible way to do 2d mesh these days is with the parts based meshing method linked to in my previous post.