The order-limit property specified by the Planner indicates to the SQL-MapReduce function that if the SQL-MapReduce function produces its output data in the order specified in 'order-limit', the function has the option to apply limit (that is, return only the first 'limit' number of rows on each vWorker).
The order and limit go together since not specifying an order results in a 'limit' number of rows being returned rather than specific rows. The order-limit property is picked up from the surrounding query by the Planner.
A typical business scenario asks for specific 'limit' number of rows. For example, the top 5 students ordered by descending order of grade. In another example, the 10 worst performing stores yields the first 10 stores when they are ordered on increasing order of revenue.