The optional size attribute defines a recommended size for visualization of an infinite drawable element such as plane, cylinder, axis, etc., or objects without explicit geometric parameters (e.g. coordinate system).
The optional transparency attribute defines the transparency property of a model entity.
Source
<xs:element name="PlaneReference" type="PlaneReferenceType" substitutionGroup="Auxiliary"><xs:annotation><xs:documentation>The PlaneReference global element is a reference plane.</xs:documentation></xs:annotation></xs:element>