  The present contribution deals with surface terms appearing immediately in
distributions and partition functions of statistical ensembles. It is shown
that all ensembles under study, including ordinary canonical and grand
canonical ensembles, involve surface terms. For a canonical ensemble both
surface and volume terms correspond to a closed system. For a grand canonical
ensemble the volume term corresponds to an open system, while the surface one -
to a closed one. Finally, for the recently introduced open statistical ensemble
the specific feature of which is the consideration of some surrounding region
both volume and surface terms correspond to an open system.
  In conclusion, surface particles at solid/fluid boundary are interpreted as
particles corresponding to number density oscillations near the surface; this
completely agrees with the earlier introduced concept of surface tension at
such a boundary.
