In addition to the predefined execution classes, you can define your own execution classes. Reasons for doing this include:
EC1, EC2, and EC3 do not accommodate all combinations of attributes that might be useful.
Associating execution objects with a particular group of engines would improve performance.
The system procedure sp_addexeclass creates a user-defined execution class with a name and attributes that you choose. For example, the following statement defines a new execution class called DS with a low– priority value and allows it to run on any engine:
sp_addexeclass DS, LOW, 0, ANYENGINE
You associate a user-defined execution class with an execution object using sp_bindexeclass just as you would with a predefined execution class.