Optional vs Mandatory

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Effects are denoted as optional if the effect has a "you can" or "you may" text directly after its activation condition. However, if the effect has "Then," afterwards, it is not considered optional even if it starts with "you may" directly after the trigger/activation condition*.

Effects that have a "your opponent may" are not considered optional, as the player controlling the effect makes the decision as to when the effect is activated. The action is optional for the opponent, but the effect is not optional for the player controlling the effect.

It is possible for optional effects without a cost to be forcefully activated and prevent them from activating again during the turn.
*There is no reason to do this as they are optional effects anyways.

*Effects that have "you may do <X> to <Y>" are considered optional even if there is a "Then," within the effect.

Examples

Example of Mandatory Effects

• [BT4-075 Blastmon]: [When Attacking] Your opponent may choose 1 of their unsuspended Digimon. If they do, switch the target of attack to that Digimon.
• [BT5-019 Shoutmon DX]: [When Digivolving] You may place 1 red Digimon card from your hand at the top of this Digimon's digivolution cards. Then, for each [OmniShoutmon]/[ZeigGreymon] in this Digimon's digivolution cards, delete 1 of your opponent's Digimon with 5000 DP or less.
• [BT6-083 Eosmon]: [On Play] You may play 1 white Tamer card with a play cost of 4 or less from your hand without paying its memory cost. Then, your opponent may play 1 Tamer card from their hand without paying its memory cost.
• [BT5-022 Bulucomon]: [Your Turn] [Once Per Turn] When one of your opponent's digivolution cards is trashed, gain 1 memory.
• [BT4-114 AncientGarurumon]: [When Attacking] [Once Per Turn] Unsuspend up to 2 of your Digimon with [Garurumon] in their (other than [KendoGarurumon]) or [Hybrid] trait.
• [BT3-027 Paildramon]: [When Attacking] [Once Per Turn] If this Digimon has [Imperialdramon] in its name, unsuspend it.

Example of Optional Effects

• [BT6-030 Gabumon - Bond of Friendship]: [When Attacking] [Once Per Turn] If you have a Tamer in play, you may unsuspend this Digimon.
• [BT2-077 Kimeramon]: [On Play] You may delete one of your other Digimon to delete 1 of your opponent's level 5 or lower Digimon.
• [BT6-083 Eosmon]: [When Attacking] You may play 1 white Tamer card with a play cost of 4 or less from your hand without paying its memory cost.
• [BT5-079 BlackWarGrowlmon]: [When Attacking] [Once Per Turn] You may delete 1 of your other Digimon to unsuspend this Digimon.
• [BT3-087 Mummymon]: [When Attacking] You may pay 3 memory to play 1 [MaloMyotismon] from your hand without paying its memory cost. Then, delete this Digimon.

Sources

• Detailed Rules Explanation
• Carddass Email English 05/10/2021: Mummymon and Cherubimon Difference
• Carddass Email English 12/10/2021: Mummymon Optional Difference
• Carddass Email English 02/11/2021: Machinedramon and Deletion Costs
• Carddass Email English 07/12/2021: Armor Purge
• Carddass Email English 15/02/2023: Force Activating Optional Effects