Hi Malibu5,
Thanks for posting in the Community!
This substitute TFN [removed] number is allowed where an employee is under the age of eighteen and earns $350 or less weekly, earns $700 or less fortnightly or earns $1517 or less monthly.
If you select the option The employee is under 18 and earns less than the current limit on the employee's Tax File Declaration screen, [removed] will be the TFN set for the employee.
For more detailed information, you can check on this article from ATO's website: Tax file number and withholding declarations.
Feel free to post again if you have any other questions. Have a great day!