The Irrevocable option ("Selection of this box will block all capability to rescind, edit or change this Forward certificate" as seen on the forward transfer form) is an optional checkbox within the Forward Certificate Transfer form available to the account holder.
If unchecked, the sending account holder can modify or withdraw the forward transfer at any time.
If the account holder decides to check the irrevocable option, the transferor will lose the ability to modify or withdraw the forward transfer. Only the GIS administrator can withdraw or modify the forward transfer after obtaining confirmation from both parties.
The reason this option is in place is to provide the transferee a bit of extra protection from the sending entity changing the terms of the forward transfer after the forward transfer is agreed upon.