You can mesh any solid model with tetrahedral solid elements. However, meshing thin models with solid elements results in generating a large number of elements since you have to use a small element size. Using a larger element size deteriorates the quality of the mesh and leads to inaccurate results. Shell meshing is the natural choice for sheet metal and thin parts. Surface models can only be meshed with shell elements.
The software generates a continuous mesh on shells sharing edges (fully or partially). For example, consider the two shells in the figure:
Meshing generates a compatible mesh along the interface and merges the nodes automatically regardless of mesh control and contact settings. If mesh control is used to apply different element sizes, the software uses the smaller size for both edges.
