private static Arity wouldEdgeVertexStepHaveSingleResult(final Step<?, ?> step) {
        final EdgeVertexStep edgeVertexStep = (EdgeVertexStep) step;
        if (edgeVertexStep.getDirection().equals(Direction.BOTH)) {
            return Arity.MULTI;
        }
        return Arity.MAY_BE_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