Rulings
Q1: What does a card with "X in its text" refer to?
It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, or Link. For example, a card with Knightmon in its text would include cards with the name DarkKnightmon and cards with the text Knightmon in their effects.
Q2: Can I place just 3 cards with Vemmon in their texts as this Digimon's bottom digivolution cards for the conditions of this card's When Digivolving effect?
No, you can't. A "by doing X" condition can't be met if only some of the required actions are performed. The conditions for this When Digivolving effect require you to place the specified number of cards as digivolution cards.
Q3: Can I return just 3 Vemmon from digivolution cards to the bottom of the deck for the conditions of this card's All Turns effect?
No, you can't. A "by doing X" condition can't be met if only some of the required actions are performed. You must return the specified number of digivolution cards to the bottom of the deck for the conditions of this card's All Turns effect.
Data Sources
Lv.6 Galacticmon BT21-062 SR Mega | Unknown | Unknown/LIBERATOR [Digivolve] [Snatchmon]: Cost 9 [When Digivolving] By placing 4 cards with [Vemmon] in their texts from your trash as this Digimon's bottom digivolution cards, you may use 1 [Ragnarok Cannon] from your hand or trash without paying the cost. [Start of Your Main Phase] Delete 1 of your opponent's Digimon. [All Turns] When this Digimon would leave the battle area, by returning 4 [Vemmon] from its digivolution cards to the bottom of the deck, it doesn't leave.
Lv.6 Galacticmon BT21-062 SR Mega | Unknown | Unknown/LIBERATOR [Digivolve] [Snatchmon]: Cost 9 [When Digivolving] By placing 4 cards with [Vemmon] in their texts from your trash as this Digimon's bottom digivolution cards, you may use 1 [Ragnarok Cannon] from your hand or trash without paying the cost. [Start of Your Main Phase] Delete 1 of your opponent's Digimon. [All Turns] When this Digimon would leave the battle area, by returning 4 [Vemmon] from its digivolution cards to the bottom of the deck, it doesn't leave.
Lv.6 Galacticmon BT21-062 SR Mega | Unknown | Unknown/LIBERATOR [Digivolve] [Snatchmon]: Cost 9 [When Digivolving] By placing 4 cards with [Vemmon] in its text from your trash as this Digimon's bottom digivolution cards, you may use 1 [Ragnarok Cannon] from your hand or trash without paying the cost. [Start of Your Main Phase] Delete 1 of your opponent's Digimon. [All Turns] When this Digimon would leave the battle area, by returning 4 [Vemmon] from its digivolution cards to the bottom of the deck, it doesn't leave.