Rulings
Q1: Which card can be played by the When Attacking effect of this card?
Cards that can be played by the When Attacking effect of this card are "Blue Lv.3 Digimon card" or "tamer card".
Data Sources
A
Official English (Asia) Card Listen.digimoncard.com
Lv.5 Mermaimon BT18-027 C Ultimate | Data | Aquabeast [When Attacking] You may play 1 blue level 3 Digimon card or 1 Tamer card from this Digimon's digivolution cards without paying the cost.
W
DCG Wikidigimoncardgame.fandom.com
Lv.5 Mermaimon BT18-027 C Ultimate | Data | Aquabeast [When Attacking] You may play 1 blue level 3 Digimon card or 1 Tamer card from this Digimon's digivolution cards without paying the cost.
JP
Official Japanese Card Listdigimoncard.com
Lv.5 マーメイモン BT18-027 C 完全体 | データ種 | 水棲獣人型 【アタック時】このデジモンの進化元から、青のLv.3のデジモンカード1枚かテイマーカード1枚をコストを支払わずに登場できる。