Hi Elias,
Receiver determination will have a sender system, sender interface and a namespace...so why for proxy the Receiver determination will be one and for IDOC it will have another receiver determination.
When you say that u have put a * in the Receiver component name it will take whatever is the config(Collabaration Agreement) available for that Sender System, Interface and namespace and proceed further.
However with ICO/Iflows u will need to have a receiver mentioned.
Regards,
Sriram