The published OCX Schema

The reference to a Surface geometry which is shared between more than one part. SurfaceRef has the following child elements:

Child	Namespace	Type	Use	Cardinality	Choice	Description
BoundingBox	ocx	ocx:BoundingBox_T	opt.	[0, 1]	False	The definition of a bounding box in space using two positions.The bounding box defines the parent extent.

SurfaceRef has the following attributes:

Attribute	Namespace	Type	Use	Fixed	Description
refType	ocx	Restriction of type xs:QName	req.	ocx:Surface	The OCX entity type which is referenced. Must be a qulaified OCX name.
localRef	ocx	xs:IDREF	opt.		A rference to a unique ID within the scope of this XML file. Ether a localRef or a GUIDRef shall be provided, or both. The schema will check this by the assertion: test=“(@localRef and not(@ocx:GUIDRef)) or (not(@localRef) and @ocx:GUIDRef) or (@localRef and @ocx:GUIDRef)”.
GUIDRef	ocx	Restriction of type ocx:guid	opt.		A globally unique ID referring to an entity in the authoring application. The sending application creates and holds the GUID. Used to establish a unique reference to an entity persistent between applications. Ether a localRef or a GUIDRef shall be provided, or both. The schema will check this by the assertion: test=“(@localRef and not(@ocx:GUIDRef)) or (not(@localRef) and @ocx:GUIDRef) or (@localRef and @ocx:GUIDRef)”.