A "Set Value" rule is used on the "Then" statement when the user wants to set a field value to something specific. For example, When Baseball_Player = Jim Then Set Value of Hitter Type to "Designated"
Set value rules are a a commonly used utility for adding meta-data to your data set. This is extremely useful for data visualization and pivoting with business events. Lets think of an other example.
Lets say that you are working with customer order data. You receive word that your best customer (Alpha) is being acquired by another company (Zeta). The acquiring company (Zeta) wants to order through your customers account, but wants it sent to a different address.
When Customer Account = Alpha
When Login = Zeta
Then Set Value of Address to Zeta's Address