An abstract feature provides a set of common properties, including id, metaDataProperty, name and description inherited from AbstractGMLType, plus boundedBy. A concrete feature type must derive from this type and specify additional properties in an application schema. A feature must possess an identifying attribute ('id' - 'fid' has been deprecated).
Database handle for the object. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs. An external identifier for the object in the form of a URI may be constructed using standard XML and XPointer methods. This is done by concatenating the URI for the document, a fragment separator, and the value of the id attribute.
Source
<complexType name="AbstractFeatureType" abstract="true"><annotation><documentation>An abstract feature provides a set of common properties, including id, metaDataProperty, name and description inherited from AbstractGMLType, plus boundedBy. A concrete feature type must derive from this type and specify additional properties in an application schema. A feature must possess an identifying attribute ('id' - 'fid' has been deprecated).</documentation></annotation><complexContent><extension base="gml:AbstractGMLType"><sequence><element ref="gml:boundedBy" minOccurs="0"/><element ref="gml:location" minOccurs="0"><annotation><appinfo>deprecated</appinfo><documentation>deprecated in GML version 3.1</documentation></annotation></element><!-- additional properties must be specified in an application schema --></sequence></extension></complexContent></complexType>