Dorumon

Q:What does "this Digimon can't leave the battle area other than by deletion" in this card's [Start of Your Main Phase] effect mean, exactly?

A:It means that this Digimon can no longer leave the battle area by processing other than deletion. Such processing includes "return to the hand/deck," "place in the security stack," and "place under another Digimon or Tamer." It only leaves the battle area by deletion.

Q:If a Digimon that's affected by this card's [Start of Your Main Phase] effect would be trashed because it has no DP, is it still prevented from leaving the battle area?

A:No, the card is trashed. Digimon without DP can't be in the battle area according to the rules, therefore upon the rule check timing, the card is considered a card that doesn't belong in the battle area and is trashed. This is why that card is trashed even if that Digimon is affected by this card's [Start of Your Main Phase] effect.