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353.2. Apply additional costs in any order.
353.2.a. Mandatory Additional Costs
353.2.a.1. Some Additional Costs specified by Passive Abilities on the card being played or another card are Mandatory , and must be paid to complete playing the card. They use the phrase "as an additional cost" and don't include the word "may." Example: A unit has the passive ability "As an additional cost to play me, kill a friendly unit." To play that unit, a player must kill a friendly unit. See rule 360. Passive Abilities for more information.
353.2.a.2. The cost imposed by the Deflect keyword is a Mandatory Additional Cost. See rule 735. Deflect for more information.
353.2.b. Optional Additional Costs
353.2.b.1. Some Optional Costs specified by Passive Abilities on the card being played or another card are Non-Mandatory , and must be paid only if the player made the choice to pay them in rule