Compound Mesh is a combination of several meshes.
To Build a compound:
From the Mesh menu select Build Compound or click button in the toolbar. The following dialog box will appear:
Name - allows selecting the name of the resulting Compound
Meshes - allows selecting the meshes which will be concatenated. They can be chosen in the Object Browser while holding Ctrl button.
Processing identical groups - allows selecting the method of processing the namesake existing on the united meshes. They can be either
United - all elements of Group1 on Mesh_1 and Group1 on Mesh_2 become the elements of Group1 on the Compound_Mesh, or
Renamed - Group1 on Mesh_1 becomes Group1_1 and Group1 on Mesh_2 becomes Group1_2. See Creating Groups for more information about groups.
Create common groups for initial meshes checkbox permits to automatically create groups of all elements of the same type (nodes, edges, faces and volumes) for the resulting mesh from the elements of the initial meshes.
You can simply unite meshes or choose to Merge coincident nodes and elements, in which case it is possible to define the Tolerance for this operation.
Example: