The edges coming into and leaving from a DecisionMergeNode must be either all object flows or all control flows.

UML constraint:

• The edges coming into and going out of a decision node, other than the decision input flow (if this exists), must be either all object flows or all control flows.

• The edges coming into and going out of a merge node must be either all object flows or all control flows.

When you get a R1200 warning on a DecisionMergeNode, remove all the flows of a specific type in order to have only one type of flow going in or out of the DecisionMergeNode.