The Merge Meshes   icon on the Geometry panel opens the Merge Meshes Utility, a convenient dialog for selecting multiple mesh elements and then merging them into a single geometry surface mesh, for use with the automatic-mesh generation option only. After surfaces are merged, you have the option to delete the original elements. You can select items from a list or by using your mouse in the 3D View, after selecting screen mode with the Select from screen   icon. This option is useful for managing geometry information imported from an STL file, where many small surfaces may have been defined. The Duplicate edge tolerance option specifies the sensitivity to merging common edges between the selected surfaces.