"Rate Formula" on element config screen - what is it for?

Summary:

Element input values have a field called "Rate Formula" - how is this used?

Content (required):

The documentation "Implementing Payroll" describes the "Rate Formula" field for element input values, but the description makes no sense to me and I can't find any other information about using this field at all.

"Select a rate calculation formula, for example to return a value from a user-defined table. This option only applies to the Primary input value for elements associated with rate definitions that have the Element method and a contributor type of Amount. If you select a formula, you must not select