A solid is the basis for 3-dimensional geometry. The extent of a solid is defined by the boundary surfaces as specified in ISO 19107:2003, 6.3.18. exterior specifies the outer boundary, interior the inner boundary of the solid.
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.
<element name="Solid" type="gml:SolidType" substitutionGroup="gml:AbstractSolid"><annotation><documentation>A solid is the basis for 3-dimensional geometry. The extent of a solid is defined by the boundary surfaces as specified in ISO 19107:2003, 6.3.18. exterior specifies the outer boundary, interior the inner boundary of the solid.</documentation></annotation></element>