While you can specify values as a default value for a control, there are times when you want a control to be updated to a value based on a different control. You can't do that with a default value, because at the time the default value is assigned there are no values, other than default values, in the other fields of the form. If you try to use another field in a default value assignment, you will receive an error. Instead of using the default value, you would specify a rule on the control you want to "trigger" the rule on.
Rules are a combination of conditions and actions that you can use to manipulate values in other controls that are based on the control to which the rules are assigned.
You can have more than one rule per control, and you can specify that those rules either run all the way through or stop when a condition is met. You can also specify the order of the rules, and rearrange them as necessary.
Here are a few examples of utilizing rules:
Assign a default tax on an order based on the city the order was sold in.
Update a contact e-mail field on an order after a customer has been chosen.
Update a ship date field to the order date field plus a couple of days when the order date is specified.
In the current form a good example of a set of rules is assigning various discounts based on the quantity ordered.
We are going to use the last example mentioned to demonstrate creating rules ...