Summary
The source and the target of an ActivityEdge must be owned by the same Activity.
Details

In this example, the ControlFlow is invalid because it has its source in the "Recruitment" Activity and its target in the "Development" Activity.
Tips
When you get a R1030 error, you can fix it by:
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moving one of the ends of the ActivityEdge into an element in the same Activity as the other end
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moving the source of the ActivityEdge into the same Activity as the target or the other way around
You can also use a CallBehaviorAction node to refer to a Behavior (i.e. an Activity, an Interaction or a CommunicationInteraction) from another Activity.