This indicates which rotation the card belongs to for tournament play
Back around 2009, New World Order came into full effect, limiting complexity at common (and increasing it at uncommon)
C Common Blackwing Armed Wing X DARK Level 6 [ Winged BeastSynchroEffect ] ATK 2300 DEF 1000 1 "Blackwing" Tuner + 1+ non-Tuner monsters If this card attacks a Defense Position monster, it gains 500 ATK during the Damage Step only, also inflict piercing battle damage to your opponent
(If this card is your chosen companion, you may put it into your hand from outside the game for as a sorcery.) When Keruga enters, draw a card for each other permanent you control with mana value 3 or greater
Duskmourn , notably, has a theme of enchantments and cards that care about enchantments, so green's ability to destroy them or blue's ability to return them to their owner's hand can prove invaluable