Hi Allen,
Thank you for reaching out to us.
You can use either of the two as both identifies an ad unit and its settings. However, when it comes to the master ad tag, I would suggest that you use the iu parameter as this is required and as suggested by the documentation below:
https://support.google.com/admanager/table/9749596?hl=en
I would suggest that you reach out to the Google Ad Manager help center if you need further assistance with regard to the ad tag parameters.
Regards,
Sherwin Diesta
IMA SDK Team