All SensorML 2.0 described objects are Geography Markup Language (GML) Features. The first three potential properties of any SensorML 2.0 described object are inherited from the GML schema, including gml:description, gml:identifier, and gml:name. In GML schema, these are all listed as optional; however, in SensorML 2.0, the gml:identifer property takes on a special purpose in that it provides a globally-unique identifier for the object described. As required by Schematron rules in SensorML 2.0, gml:identifier must be present and must have the codespace="UID" attribute. The value of the gml:identifier can be a unique code generated by some UID engine, or perhaps a URN that assures its uniqueness.
The other GML properties provide for an inline textual description within the gml:description property and a common name for the object described using the gml:name. For longer descriptions, GML also allows for a reference to an external description using gml:descriptionReference, although one should perhaps consider the sml:documentation property (discussed later) as a more appropriate place for referencing an external document describing the object.
Also shown here in this XML example snippet is the sml:keywords property, which as one would expect provides for a list of keywords that might assist one in discoverying this particular object (process, sensor, actuator, etc.). The sml:KeywordList element has an optional codespace attribute which can be used to reference, through a resolvable URL, a specific online dictionary or collection of terms from which these keywords can be selected.