Q1: What does this card's On Play When Digivolving effect do, exactly?

Choose 3 of your opponent's Digimon, and the effect returns the top 5 cards from each stack to the top of the deck.

Q2: Which player chooses the order in which the cards are returned to the deck for this card's On Play When Digivolving effect?

The player who activated this effect chooses.

Q3: What happens when a Digimon I select with this card's On Play When Digivolving effect has five or less cards in its stack?

Return cards to the deck until there are no more cards stacked on top. When only 1 card remains, it is not a stacked card, so no more cards are returned to the deck.

Q4: What happens if this card's On Play When Digivolving effect returns stacked cards to the deck and the Digimon becomes a Digimon without DP?

It's trashed upon the rule check timing. This trashing isn't considered trashing from the battle area.

Q5: What happens if this card's On Play When Digivolving effect returns stacked cards to the deck and the Digimon becomes an Option card?

It's trashed upon the rule check timing. This trashing isn't considered trashing from the battle area.

Q6: When does "when your effects add to decks" trigger?

This effect triggers when a card is placed in the deck from any area other than the deck. It triggers regardless of whether the card is placed on the top or the bottom of the deck. However, it does not trigger when a card is revealed from the deck and returned.

Q7: When an effect such as "Draw 1. Then, if your hand has 6 or more cards, return 1 card in your hand to the bottom of the deck" removes cards from the deck then adds cards to it, does the effect "when your effects add to decks" trigger?

Yes, it does. It also triggers when an effect adds cards to the deck then removes them.

Q8: When one of my effects adds cards to my opponent's deck, does this card's "when your effects add to decks" effect trigger?

Yes, it does.

Q9: What kind of card 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), digivolve requirements, DNA Digivolve, DigiXros requirements, Burst Digivolve, App Fusion, Link, or Assembly requirements. 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.