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There is no need to customize - configuring the Relationship constraint will give you this behavior. Once you configure you will only be able to relate the defined subtypes and not other subtypes. The UIs used while relating will only let you select Docs of the appropriate type.
Please check out Relationship Constraints. They will allow you to configure which types of objects can be used in relationships. For Part to Doc be sure to look at whether its a Describes or Reference relationship you want to configure.
Check out this section of help: Relationship Constraints Tab
There is no need to customize - configuring the Relationship constraint will give you this behavior. Once you configure you will only be able to relate the defined subtypes and not other subtypes. The UIs used while relating will only let you select Docs of the appropriate type.