The Message Builder Node accepts a string of text and appends each text of that string to the message each time the node is used. This can be useful when your process loops through records and records any error messages for each record. It’s also good for notification messages for multiple records.
The Variable name is what will be referenced when you want to use the built message later in the process.
Reference the variable name just like any other variable name. All nodes after the message builder will have access to the message builder variable.