In GML the points of a LineString are defined by a coordinate list and are not defined by GML Points. A LinearRing is a LineString that is both closed and simple. In GML, the points of a LinearRing are defined by a coordinate list and are not defined by GML Points. A Polygon is a planar Surface, defined by 1 exterior boundary and 0 or more interior boundaries. Each interior boundary defines a hole in the Polygon.
Polygons are topologically closed.
The boundary of a Polygon consists of a set of LinearRings that make up its exterior and interior boundaries. Note that these are captured in GML via the eboundaryis and iboundaryis properties of the Polygon. No two rings in the boundary cross, the rings in the boundary of a Polygon may intersect at a Point but only as a tangent. The Interior of every Polygon is a connected point set. The Exterior of a Polygon with 1 or more holes is not connected.