gml:DerivedCRS is a single coordinate reference system that is defined by its coordinate conversion from another single coordinate reference system known as the base CRS. The base CRS can be a projected coordinate reference system, if this DerivedCRS is used for a georectified grid coverage as described in ISO 19123, Clause 8.
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="DerivedCRS" type="gml:DerivedCRSType" substitutionGroup="gml:AbstractGeneralDerivedCRS"><annotation><documentation>gml:DerivedCRS is a single coordinate reference system that is defined by its coordinate conversion from another single coordinate reference system known as the base CRS. The base CRS can be a projected coordinate reference system, if this DerivedCRS is used for a georectified grid coverage as described in ISO 19123, Clause 8.</documentation></annotation></element>