The Big Picture of Calculation Rule Eligibility Rules
You can use eligibility rules to define the criteria when a calculation rule should be applied (or skipped); for example,
• You may have a calculation rule that generates an employee discount. This calculation rule may have eligibility criteria that requires the person to have a given characteristic type and value (this assumes you use a person characteristic to identify employees) before the calculation rule is applied.
• You may have a calculation rule that applies a surcharge if the customer uses 25% more than they used in the same period in prior year.
Note: 
Eligibility rules are optional. If you add eligibility rules to a calculation rule, the calculation rule will be skipped or applied per the instructions in the rules. If you don't add eligibility rules to a calculation rule, however, the calculation rule will be executed – this is because calculation rules are eligible by default
The topics in this section describe eligibility rules.