The main difference between the Contributing Content and Content associations are that Contributing Content will "contribute" to the WTPart through Attribute propagation. For example, a Contributing Content associated CAD Document will push attribute values to the WTPart (depending on how Auto-Associate is configured).
If I remember correctly the various association types are described in detail in the help files but here are my recommendations for use;
Owner - use as default when there is one CAD Document that describes a WTPart.
Contributing Content - use when you do not want the CAD Structure to define the WTPart structure. As an example, a weldment where the designer models the welded component as an assembly with each material piece being modeled as a part and you do not want material_01, material_02 etc. in the structure.
Content - use to associate secondary models in use cases such as a Multi-CAD environment where versions of the same thing exist in different file formats (Creo/SW/UG/Catia etc.). Your preferred application model would have the Owner association, and other would be Content so assemblies of all formats will build an appropriate WTPart Structure, but only the preferred (or primary) application will drive Attributes.