Creo Options Modeler > References > Data Management > Managing Parts and Documents > About Associating Parts and Documents
  
About Associating Parts and Documents
Windchill creates structural relationships between WTParts and related CAD documents and maintains the association until it is no longer linked.
The association allows implicit checkout of CAD document and WTPart objects, if they are not already checked out. In the case of a CAD document, checkout is for meta-data checkout only; no content is downloaded during this checkout action. The property sheets of the CAD document and WTPart display the current relationship between a WTPart and a CAD document, even if the CAD document is not checked in. Before a CAD document is associated, the Creo application performs an implicit (automatic) checkout.
 
Any object checked out implicitly remains checked out even if the association fails to check out the other object.
After you associate WTparts with all CAD documents in an assembly, you can see the association; however, Uses links between the WTparts will not be visible until after check in, when the Windchill build rule constructs the Uses links.
It is recommended that you associate a WTPart with a CAD document at the time of object creation, not when you create a structure. A CAD document is said to describe an enterprise part to be included in the bill of materials, and an enterprise part is described by a CAD document.