gml:AffineCS is a two- or three-dimensional coordinate system with straight axes that are not necessarily orthogonal. An AffineCS shall have two or three gml:axis property elements; the number of property elements shall equal the dimension of the CS.
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.
Source
<element name="AffineCS" type="gml:AffineCSType" substitutionGroup="gml:AbstractCoordinateSystem"><annotation><documentation>gml:AffineCS is a two- or three-dimensional coordinate system with straight axes that are not necessarily orthogonal. An AffineCS shall have two or three gml:axis property elements; the number of property elements shall equal the dimension of the CS.</documentation></annotation></element>