public Arity computeArity(final Arity operandArity) {
            final Arity inputArity = this;
            if (inputArity.getPriority() * operandArity.getPriority() > 1) {
                return MULTI;
            } else if (inputArity.equals(MAY_BE_SINGULAR) || operandArity.equals(MAY_BE_SINGULAR)) {
                return MAY_BE_SINGULAR;
            }
            return 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