The Being Ordered refers to the data is stored in a certain order. Every piece of data and every member have their absolute or relative numbers. You can access the data with these serial numbers and operate on the data conveniently, such as locating, ranking, or sorting.
With serial numbers, you can access to the member of sets more easily.
Get the sales value field of the last record: sales.m(-1).(amount)
Get records whose sales value are over 1000: sales.pselect@a(amount>1000)
Get the record with the highest sales amount: sales.pmax@a(amount)