thank you very much for your answer but unfortunately that does not work.
I tried this way before I tried the way over geometry primitives.
When you convert a step-file into stl, also if you have different body volumes inside your model, you only get back one solid stl file which contains all geometries.
As I tried the way with stl faces for defining MRF zones the meshing operation stops with error messages.
In the documentation it is also written that is only possible to use volumes:
(It should be noted that to create a MRF cell zone the assigned entity should only be a ‘volume or volume set‘ and Not a face or face set. Further, please make sure that the name of the cell zone does not start with a number and does not contain spaces. For example, valid names would be “MRFZone”, “MRFZone_1”, “rotating_cells” and Not “1MRFZone”, “MRF zone”.)