The links between , EPM and part are different and more complicated
the given link WTPartDescribeLink is using roles as below
| wt.doc.WTDocument |
describedBy |
wt.part.WTPartDescribeLink |
describes |
wt.part.WTPart |
The link between parts and cad can be for different associations (image,owner,content..) and it is created by actions like build structure (starting from CAD assembly or from Part BOM) or by auto-associate ..
One of the tables that holds cad-part is EPMBuildHistory
EPMBuildHistory has two roles one is to WTPart, the other is to EPM

For content association we have below table

Hope this helps,