Rulings
Q1: I have this Digimon in play, and attack with a black Digimon with 12000 DP, whose When Attacking effect grants itself +1000 DP, bringing its total DP up to 13000. Does this Digimon's effect allow that black Digimon to have Piercing during the battle?
Yes. This Digimon's effect becomes active as soon as your black Digimon reaches 13000 DP or more. Your black Digimon will conduct its battle with Piercing.
Q2: I attack with a black Digimon with 12000 DP, whose When Attacking effect grants itself +1000 DP, bringing its total DP up to 13000. If this Digimon is in its digivolution cards, does this Digimon's inherited effect allow the black Digimon to have Security Attack +1 during its battle?
Yes. This Digimon's inherited effect becomes active as soon as your black Digimon reaches 13000 DP or more. Your black Digimon will conduct its battle with Security Attack +1.
Q3: This card is in the digivolution cards of a black Digimon with 13000 DP, which I use to attack my opponent. The first card flipped over during the security check causes the black Digimon to {{DeDigivolve}}, resulting in it having 12000 DP or less. Does the black Digimon still get extra security checks from Security Attack +1?
This card's inherited effect is a Your Turn effect, so your black Digimon will lose Security Attack +1 as soon as it drops below 13000 DP. If the Digimon has no other effects to increase the number of security checks, the attack will end without performing any further checks.
Data Sources
Lv.3 Sunarizamon P-033 P Rookie | Virus | Reptile [Your Turn] All of your black Digimon with 13000 DP or more gain <Piercing>. (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.) --- Inherited: [Your Turn] While this Digimon is black and has 13000 DP or more, this Digimon gains <Security Attack +1>. (This Digimon checks 1 additional security card.)
Lv.3 Sunarizamon P-033 P Rookie | Virus | Reptile [Your Turn] All of your black Digimon with 13000 DP or more gain <Piercing>. (When this Digimon attacks and deletes an opponent's Digimon and survives the battle, it performs any security checks it normally would.) --- Inherited: [Your Turn] While this Digimon is black and has 13000 DP or more, this Digimon gains <Security Attack +1>. (This Digimon checks 1 additional security card.)
Lv.3 Sunarizamon P-033 P
Rookie | Virus | Reptile
[Your Turn] All of your {{Colour|black}} Digimon with 13000 DP or more gain <Piercing>.
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Inherited: [Your Turn] While this Digimon is {{Colour|black}} and has 13000 DP or more, this Digimon gains <Security A. +1>.Lv.3 スナリザモン P-033 P 成長期 | ウィルス種 | 爬虫類型 【自分のターン】黒のDP13000以上の自分のデジモン全ては≪貫通≫（このデジモンがアタックしたバトルで相手のデジモンだけを消滅させたとき、このデジモンはセキュリティをチェックする）を得る。 --- Inherited: 【自分のターン】このデジモンが黒のDP13000以上の間、このデジモンは≪セキュリティアタック+1≫（このデジモンがチェックするセキュリティの枚数+1）を得る。