private static Arity getOutputArityBehaviorForTokenTraversal(final Traversal traversal) {
        // if its a token traversal then
        // id is Definitely singular

        final TokenTraversal tokenTraversal = (TokenTraversal) traversal;
        if (tokenTraversal.getToken().equals(T.id)) {
            return Arity.DEFINITELY_SINGULAR;
        } else if (tokenTraversal.getToken().equals(T.label)) {
            return STEP_TO_ARITY_FUNCTION_MAP.get(LabelStep.class.getSimpleName()).apply(null);
        }
        return Arity.MULTI;
    }
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