Formula look-ups allow you to extend the simple AD look-up functions, and use a C# script to manipulate the data and return a string.
Formula
A C# script that will be executed.
The formula is parsed in the same way look-ups are in the body of the signature.
In the above example the (!Fax!) look-up will be calculated and the results will be inserted between the quotes to generate a string. This text is then used in the formula to determine if the phone number and description should be inserted into the signature.