gml:SRSReferenceGroup 

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.

Attribute Group Information

Attributes

QName Type Fixed Default Use Inheritable Annotation
axisLabels gml:NCNameList optional
srsDimension positiveInteger optional
srsName anyURI optional
uomLabels gml:NCNameList optional

Used By

Complex Types gml:AbstractGeometryType, gml:DirectPositionListType, gml:DirectPositionType, gml:EnvelopeType

Source 

<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>