By using this function, you can force a message to be written to the Logs. That message in the Logs will display the record number or ID that has been transferred to your destination system.
You can use this function as part of the output of an IF function, but you can also use it independently.
##IF(EQUALS("Some_Field",'ValueA'),MESSAGE("Some_field has value A"),'')+"Some_Field"
The formula will return the value of the field "Some_Field", but in addition to this it will Log a Message if the field value is 'ValueA'
##"No_"+MESSAGE("No_")
The above formula is added under Table Link (key fields) -> Source Field so that you can retrieve the Number (No_) of the records transferred from the source system and you will see a full list of record numbers in your Logs.