Ports 
A port indicates which attribute bundles can be used with which graphics proxies.
The customizing objects are assigned to the ports in the graphics proxies. Only some combinations of ports and customizing objects are possible. Not every customizing object can be assigned to every port. For more information on possible combinations see the class documentation for the appropriate graphics proxy.
The ports in a graphics proxy are arranged in a hierarchy that determines where the proxy looks for customizing objects. For more information, see Hierarchy of Ports.
Structure
The ports are defined as constants in IF_GRAPHIC_PROXY. Their names have the prefix CO_PORT_ they are all documented in the class builder.