MESSAGE(string);

Using this function you can force a message to be written to the Log.

It makes most sence to use this function as part of the output of an IF function.

Example

IF(EQUALS("Some_Field",'ValueA'),MESSAGE("Some_field has value A"),'')+"Some_Field"

The formula will just return the value of the field "Some_Field", but in addition to this it will Log a Message if the fields value is 'ValueA'