geometryComplexes.xsd

See ISO/DIS 19136 12.2.
Geometric complexes (i.e. instances of gml:GeometricComplexType) are closed collections of geometric primitives, i.e. they will contain their boundaries. 
A geometric complex (gml:GeometricComplex) is defined by ISO 19107:2003, 6.6.1 as "a set of primitive geometric objects (in a common coordinate system) whose interiors are disjoint. Further, if a primitive is in a geometric complex, then there exists a set of primitives in that complex whose point-wise union is the boundary of this first primitive."
A geometric composite (gml:CompositeCurve, gml:CompositeSurface and gml:CompositeSolid) represents a geometric complex with an underlying core geometry that is isomorphic to a primitive, i.e. it can be viewed as a primitive and as a complex. See ISO 19107:2003, 6.1 and 6.6.3 for more details on the nature of composite geometries.
Geometric complexes and composites are intended to be used in application schemas where the sharing of geometry is important.