The Mesh Controls node will display different options, depending on whether you selected to import geometry only and use the automatic-mesh generation option, or opted to read in a pre-defined body-fitted mesh. For the automatic-mesh generation, some mesh-size criteria and refinement controls must be specified, as well as a material point within the computational domain. From these parameters and from the boundary conditions specified later in the Workflow tree, the mesh-generation is performed automatically during simulation. For the body-fitted mesh option, a built-in algorithm is used to adjust the pre-defined mesh, when a moving-piston boundary is defined in that mesh.