The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
A triangulated surface is a polyhedral surface that is composed only of triangles. There is no restriction on how the triangulation is derived.
trianglePatches encapsulates the triangles of the triangulated surface.