Rule 465.2.c.6
A player must obey all requirements and restrictions on damage assignment if able. Example: A player is assigning damage to the following units: a unit with Tank ("I must be assigned combat damage first."); a unit with Backline ("I must be assigned combat damage last."); and another unit without any abilities. That player must assign combat damage first to the unit with Tank, then to the unit with no abilities, then to the unit with Backline.