For EPMDocument, the where used tab only contains the EPMDocuments from ProjectLink if the EPMDocument is opened in ProjectLink. If the where used tab is opened from an EPMDocument in PDMLink, it contains only the PDMLink assemblies.
However for WTParts, opening the part in either PDMLink or ProjectLink gives correctly all where used WTParts from both PDMLink and ProjectLink.
The where used for the CADDocument should display where used for both projects and products as it has been done for WTPart. The current behaviour is very confusing as it is different for EPMDocuments and WTParts.