private static Arity getOutputArityBehaviorForTraversalMapStep(final Step step) {
        if (!(step instanceof TraversalMapStep)) {
            throw new IllegalArgumentException(String.format("Expected TraversalMapStep but got '%s'", step.getClass().getSimpleName()));
        }
        final TraversalMapStep mapStep = (TraversalMapStep) step;
        return getOutputArity((Traversal<?, ?>) mapStep.getLocalChildren().get(0));

    }
This enum value tells that the output arity of the step is multiple.
For example: out(), in() unfold etc.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/util/StepOutputArityPredictor.java