Today, cards with the “E” Mark at the bottom of the card were phased out of standard legal play, leaving only cards with an “F” mark, “G” mark and “H” mark in standard legal play. Attached below is an example of a “G” mark card, shown at the bottom left of the card.
https://archives.bulbagarden.net/media/upload/a/af/BruteBonnetParadoxRift123.jpg
this is called a Regulation Mark and helps knowing when a card will rotate. Cards with a regulation D (cards from 2020) are rotating out of standard format today, cards from regulation E will rotate in 2024, cards from regulation F rotate in 2025 and so on.
The reason card games rotate their older cards out is to keep the format fresh and prevent any unwanted interactions.
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