826.1. Backline is a Passive Ability keyword.
826.2. It is present on Units.
826.3. It is functionally short for "I must be assigned lethal damage after any other unit with the same controller as me that does not have [Backline] during the Combat Damage step."
826.4. It alters how players can elect to assign combat damage during combat.
826.5. Multiple Instances of Backline are redundant.
826.6. Backline, and whether or not a unit has Backline, is a characteristic of the Unit and may be checked or referenced by other Game Effects.