Using this function you can force a Transfer to end with a specific error message.
It makes most sence to use this function as part of the output of an IF function.
Example:
IF(EQUALS("Some_Field",'ValueA'),ERROR("Some_field has value A - stopping transfer"),'')+"Some_Field"
The formula will normally just return the value of the field "Some_Field", but if the fields value is 'ValueA' then the whole transfer will stop and this error message will be logged.