private static Arity getOutputArity(final Traversal<?, ?> traversal) {
        if (traversal == null) {
            throw new NullPointerException("traversal is marked non-null but is null");
        }

        return getOutputArity(traversal, Arity.DEFINITELY_SINGULAR);
    }
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