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:complexType name="PlaneReferenceType"><xs:annotation><xs:documentation>The PlaneReferenceType defines a plane with user defined attributes and direct references; the plane may be used as an auxiliary plane.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="AuxiliaryPlaneBaseType"/></xs:complexContent></xs:complexType>