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A decision node is a construct in SysML that represents a branching point in an activity diagram where alternative paths are chosen based on some conditions. The guards are expressions that specify the conditions for each outgoing edge from the decision node. A criterion that indicates that a decision node construct is well-formed is that the guards cover all possibilities and are mutually exclusive, meaning that for any input value, exactly one guard evaluates to true and all others evaluate to false. This ensures that there is no ambiguity or conflict in choosing the next path in the activity.