gml:Face 

gml:Face represents the 2-dimensional topology primitive.
The topological boundary of a face (gml:directedEdge) consists of a sequence of directed edges. If provided, the aggregationType attribute shall have the value "sequence".
The optional coboundary of a face (gml:directedTopoSolid) is a pair of directed solids which are bounded by this face. A positively directed solid corresponds to a solid which lies in the direction of the negatively directed normal to the face in any geometric realisation. 
A face may optionally be realised by a 2-dimensional geometric primitive (gml:surfaceProperty).

Element Information

Model

gml:metaDataProperty* , gml:description{0,1} , gml:descriptionReference{0,1} , gml:identifier{0,1} , gml:name* , gml:isolated* , gml:directedEdge+ , gml:directedTopoSolid{0,2} , gml:surfaceProperty{0,1}
Children: gml:description, gml:descriptionReference, gml:directedEdge, gml:directedTopoSolid, gml:identifier, gml:isolated, gml:metaDataProperty, gml:name, gml:surfaceProperty

Attributes

QName Type Fixed Default Use Inheritable Annotation
aggregationType gml:AggregationType optional
gml:id ID required 
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.
universal boolean false optional 
If the topological representation exists an unbounded manifold (e.g. Euclidean plane), a gml:Face must indicate whether it is a universal face or not, to ensure a lossless topology representation as defined by Kuijpers, et. al. (see OGC 05-102 Topology IPR). The optional universal attribute of type boolean is used to indicate this. NOTE The universal face is normally not part of any feature, and is used to represent the unbounded portion of the data set. Its interior boundary (it has no exterior boundary) would normally be considered the exterior boundary of the map represented by the data set.

Used By

Complex Types gml:DirectedFacePropertyType, gml:FaceOrTopoSolidPropertyType

Source 

<element name="Face" type="gml:FaceType" substitutionGroup="gml:AbstractTopoPrimitive">
  <annotation>
    <documentation>gml:Face represents the 2-dimensional topology primitive. The topological boundary of a face (gml:directedEdge) consists of a sequence of directed edges. If provided, the aggregationType attribute shall have the value "sequence". The optional coboundary of a face (gml:directedTopoSolid) is a pair of directed solids which are bounded by this face. A positively directed solid corresponds to a solid which lies in the direction of the negatively directed normal to the face in any geometric realisation. A face may optionally be realised by a 2-dimensional geometric primitive (gml:surfaceProperty).</documentation>
  </annotation>
</element>

Sample