Because some work may have variable resource requirements, you
can define multiple periods for a service class. Periods are
a way of defining different goals for work depending on the amount
of resources the work consumes. Typically, periods are used to give
shorter transactions more aggressive goals and to give longer running
work of the same type less aggressive goals. If you have multiple
periods, each period except the last has a duration. Duration is
the amount of resources 
(including all processor
types)
, in service units, that the work consumes before it is
switched to the goals of the next period.
You can also group work into a service class based on resource requirements. If you have a group of batch work that can consume vast amounts of resources, and you want to limit it, you can define a service class and assign it to a resource group with a maximum amount of capacity. If the work exceeds that capacity, workload management slows the execution rate. Also, if a certain group of work needs a minimum amount of processor capacity, you can set up a service class and assign it to a resource group.