This type of data object is a triangle mesh represeting a surface or, more precisely, a two-dimensional manifold that is closed and orientable. Typically, surface meshes are produced by modifiers such as Construct surface mesh, Create isosurface or Coordination polyhedra.
The appearance of the surface mesh is controlled by the corresponding surface mesh visual element.
What is special about surface meshes is that they may be embedded in periodic domains, i.e. simulation cells with periodic boundary conditions. That means triangles of a surface mesh can connect vertices on opposite sides of a simulation box and wrap around correctly. OVITO takes care of computing the intersections of the periodic surface with the box boundaries and automatically produces a non-periodic representation of the triangle mesh when it comes to visualizing the surface.
As surface meshes are closed orientable manifolds, one can define an interior and an exterior region of space that are separated by the surface manifold. For example, if the surface mesh was constructed by the Construct surface mesh modifier from a set of particles, then the region enclosed by the surface represents the "solid" region and the outside region is the one containing no particles.
Sometimes there is no interior region and the exterior region is infinite and fills all space. In this case the surface mesh is degenerate and consists of no triangles. The opposite extreme is also possible in periodic domains: The interior region extends over the entire periodic domain and there is no outside region. Again, the surface mesh will consist of zero triangles in this case.
A surface mesh can be exported from OVITO in the form of a conventional triangle mesh. For this, a non-periodic version is produced by truncating triangles at the domain boundaries and generating "cap polygons" to fill the holes that occur at the intersection of the interior region with the domain boundaries. To export the mesh, use OVITO's file export function and select the VTK output format.