3 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Can I choose to not activate this card's <On Play> effect?
Yes, the effect reads "By placing this Digimon under 1 of your other Digimon [...] as its bottom digivolution card," so you may also choose to not place it as the bottom digivolution card. In such cases, it remains in the battle area as a Digimon like normal.
Ruled Mar 28, 2024
When I reveal a card with the [Legend-Arms] and a play cost of 7 or less from my deck for this card's <On Play> effect, can I choose to not play that card?
Yes, the effect reads "you may play it," so you may also choose to not play it. In such cases, you add it to your hand like any other card.
Ruled Mar 28, 2024
I revealed a card with the [Legend-Arms] and a play cost of 7 or less from my deck for this card's <On Play> effect] when I had a [BT9-047]/[Pomumon]. What happens to that card?
It can't be played due to [BT9-047]/[Pomumon]'s effect, so you add it to your hand like any other card.
Ruled Mar 28, 2024
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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.