Menu: Geometry->Separate connected components
Separates connected components from shown volume and surface meshes.
For each mesh detects groups of connected elements and stores them in separate sets.
For instance a single volume mesh which corresponds to three unconnected spheres will be separated into three sets each one containing the volume elements of a single sphere. The original mesh is left untouched. By default a maximum of 50 connected groups will be separated into separated sets. This limit can be modified with the variable Utilities --> Variables --> PostMaxNumComponents on the main's window the right menu.