Let's say that a person's eligibility for a product is based on whether that person's total income is below a certain limit. This limit is revised periodically in line with costs of living and inflation.
The rules for eligibility do not vary as such; however the income limit does vary over time, and so a rules designer models the income limit as a rate table, rather than "hard-coding" the income limit directly in the rule set. This approach allows the income limit to vary independently of rules; changes to the income limit can be effected by publishing changes to the rate table, rather than by changing the rule set. In particular, the rule set itself does not need to be retested when the income limit changes (but of course any users changing rate tables should satisfy themselves that the change being made will have the desired effect, possibly by first trialling the change in a test environment).
The eligibility rules will use the rate expression to retrieve the value of the required rate, and use this value to determine whether a person's total income is within the bounds for eligibility.
John Smith makes a claim for benefit. John's total income varies as follows:
In parallel, the agency varies the income limit applied to eligibility calculations as follows:
John's eligibility varies not only according to the variations in his total income level, but also according to the varying income limit rate: