gml:ImageCRS is an engineering coordinate reference system applied to locations in images. Image coordinate reference systems are treated as a separate sub-type because the definition of the associated image datum contains two attributes not relevant to other engineering datums.
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="ImageCRS" type="gml:ImageCRSType" substitutionGroup="gml:AbstractSingleCRS"><annotation><documentation>gml:ImageCRS is an engineering coordinate reference system applied to locations in images. Image coordinate reference systems are treated as a separate sub-type because the definition of the associated image datum contains two attributes not relevant to other engineering datums.</documentation></annotation></element>