The attribute group SRSReferenceGroup is an optional reference to the CRS used by this geometry, with optional additional information to simplify the processing of the coordinates when a more complete definition of the CRS is not needed.
In general the attribute srsName points to a CRS instance of gml:AbstractCoordinateReferenceSystem. For well-known references it is not required that the CRS description exists at the location the URI points to. 
If no srsName attribute is given, the CRS shall be specified as part of the larger context this geometry element is part of.

<attributeGroup name="SRSReferenceGroup">
    <annotation>
        <documentation>The attribute group SRSReferenceGroup is an optional reference to the CRS used by this geometry, with optional additional information to simplify the processing of the coordinates when a more complete definition of the CRS is not needed.
In general the attribute srsName points to a CRS instance of gml:AbstractCoordinateReferenceSystem. For well-known references it is not required that the CRS description exists at the location the URI points to. 
If no srsName attribute is given, the CRS shall be specified as part of the larger context this geometry element is part of.</documentation>
    </annotation>
    <attribute name="srsName" type="anyURI" />
    <attribute name="srsDimension" type="positiveInteger" />
    <attributeGroup ref="gml:SRSInformationGroup" />
</attributeGroup>