Usually, that means Commander, but sometimes it can mean being played in Magic's eternal formats: Legacy and Vintage
Card not found, on the other hand, saw a lot of Standard play and is a stable of Red Deck Wins in both Explorer and Pioneer making it a key card with this mechanic
At level 0, they have the power and toughness that appears in the top text box
Creatures you control get +1/+0 and have vigilance as long as you control three or more creatures.Creatures you control also get +1/+0 and have trample as long as you control six or more creatures.Creatures you control also get +1/+0 and have double strike as long as you control nine or more creatures
A 4/4, whenever the Arsonist attacks, it deals one damage to up to one player, one creature, and one planeswalker
A complete Mega Evolution master set contains 320 cards 188 main set and secret rare cards plus 132 reverse holos (one for each Common, Uncommon, Rare, and Double Rare)