Modern Japanese sets (Sword & Shield, Scarlet & Violet, Mega era) use letter codes SR, SAR, AR, UR while older cards used star symbols
The defending player can then choose whether to play a Counter card from their hand, trashing a character card in order to add the specified Counter value to the defending card's power or an event card to activate the effect as explained on the card
Do any include Palladium Myr, Endbringer, or Ulamog's Dreadsire
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Creatures that generate value again and again
It's probably the best commander for a surveil deck in EDH, but its good as a 3-drop in many other Sultai graveyard decks, though it needs some synergies to be good