A diagram harness part cannot be opened in Creo directly without having the assembly. Windchill allows you to add part to WS and does not pull the required assembly with it. Currently it is a relation reference in Windchill and it behaves as a part added in a normal assembly. Aside from designating a diagram harness part with a naming convention it would be a great enhancemnt if Windchill enforced the relationship of the part to the assembly.
Please vote if you think this to be something you would like to have.