Q1: What does "original DP" refer to?

The DP value printed on a card is its original DP. This card's effect temporarily allows you to treat the DP value printed on an opponent's Digimon as if it were 3000.

Q2: I use this card's effect to change the original DP of an opponent's Digimon to 3000. I then use ST3-11 Seraphimon's effect to give an opponent's Digimon -4000 DP. If the opponent's Digimon drops to 0 DP, is it deleted?

Yes, when a Digimon's DP becomes 0, it's deleted.

Q3: This card's effect has set a Digimon's original DP to 3000. If ST1-13 Shadow Wing is used to give that Digimon +3000 DP, is its DP now 6000?

Yes, effects that increase or decrease DP work as normal, raising the Digimon's DP to 6000.

Q4: If I use this card's effect on a Digimon that's already getting +1000 DP from an effect, does its DP become 3000?

No, its original DP becomes 3000. 3000 DP and 1000 DP (from the effect) add up to 4000 DP.

Q5: I digivolve a Digimon while this card's effect is active. What happens to the Digimon's DP?

Even after digivolving, the Digimon's DP is treated as 3000 until the end of the turn.

Q6: If I set a Digimon's original DP with this card, and then use another card to set its original DP again. What happens?

The effect that was used last takes precedence.

Q7: Can I use BT1-105 Blast Fire to decrease the DP of a Digimon with "DP cannot be reduced"?

Yes, the effect prevents the reduction of DP. But it does not prevent its original DP from being set to another number.

Q8: If after using this card's effect to set my opponent's Digimon's DP to 3000 and it gets De-Digivolve and becomes a Level 2 Digimon with no DP. Does this card's effect set the Level 2 Digimon's DP to 3000 and stop it from being trashed?

No, the Level 2 Digimon has no DP and so its DP can't be changed and it will still be trashed.