Writing Formulas and Mandatory Rules
You cannot insert other formula user-defined fields into the formula.
Formula user-defined fields calculate values based on other basic or user-defined fields. You can write formulas consisting of arithmetic expressions, alphanumeric, date and numeric functions, as well as conditional expressions.
When you work with Address Book entries, Opportunities, or Customer Service cases, you can specify that user-defined fields are mandatory if a rule is matched. Rules for mandatory user-defined fields are similar to the formulas in formula user-defined fields.
A mandatory rule consists of a comparison statement. It may compare a field with a value or with another field. Or it may compare formulas using the same syntax as formula user-defined fields. Comparison statements can contain any of the elements included in formulas, including alphanumeric, date, and numeric functions.