If you want to supply parameters in a process link you need to change the start of the linked process to Internal event and choose a business entity for the start parameters. The same thing applies if you want to return values from a process, then you have to change the end element to Internal event.
As soon as you have done this, back in the parent process in the Link configuration dialog, the drop down on the bottom becomes active and you can choose from all internal starts of the child process.
After clicking Next you can configure what parameters to assign and return (you can only return values if you configure the link as synchronous). When you use mapped business entities there is a virtual field that's named the same as the business entity. If you map this field the child process selects the same business entity that is selected in the parent (but that only works with single entities and not collections).
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