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.
The uom attribute provides an identifier of the unit of measure used for this coordinate system axis. The value of this coordinate in a coordinate tuple shall be recorded using this unit of measure, whenever those coordinates use a coordinate reference system that uses a coordinate system that uses this axis.
Source
<element name="CoordinateSystemAxis" type="gml:CoordinateSystemAxisType" substitutionGroup="gml:Definition"><annotation><documentation>gml:CoordinateSystemAxis is a definition of a coordinate system axis.</documentation></annotation></element>