741.1. Tank is a Passive Ability keyword.
741.1.a. It is present on Units.
741.1.b. It is functionally short for "I must be assigned lethal damage before any other unit with the same controller as me that does not have [Tank] during combat resolution."
741.1.c. It alters how players can elect to assign combat damage during combat.
741.1.c.1. Players must still assign lethal damage to a unit before moving to the next when assigning their damage.
741.1.c.2. If more than one unit with Tank is present with the same controller in Combat, damage may be assigned to any of them. Units without Tank are invalid assignments until all units with Tank have lethal damage assigned to them.