Void Sims (aka Ocean Sims or Empty Sims or Openspaces) are sims that (usually) contain only water and do not allow building. They exist primarily to separate normal sims, yet still allow physical travel between them. Despite the fact that Linden void sims generally have building disabled, void sims can in fact support the feature, as well as dry land, just as a normal sim can. They simply have lower prim limitations (1875 instead of 15000) and probably more easily suffer performance degradation under heavy load.