Right next to the set image and card number, is a little D in a white box with a black outline. What does this mean? ^ Example
These mysterious pentagon shaped marks are called a Regulation Mark and were first introduced during the first ever Sun & Moon sets of Japan. Japan currently has a different format system than our International format system.
Back in September 2019, Japan announced their first rotation for the Regulation Marks.
They had announced that all cards with the Regulation Mark A would rotate out. Sets beginning with the Regulation Marks B, C and D (and later on) will be legal to play in their respective Standard format for this current season.
Rough explanation on how this system works:
Some reprints in Japan during later sets, are straight reprints (with possible new artwork) from their respective set. These work almost the same as the Alternate Art symbol cards we have here. They won't survive another rotation unless they will get another Regulation Mark that will stay in their Standard Format.
(Rough conversion to our English sets)
Regulation Mark A = Sun & Moon Base Set to Crimson Invasion Regulation Mark B = Ultra Prism to Lost Thunder Regulation Mark C = Team Up to Cosmic Eclipse Regulation Mark D = Sword & Shield TCG to future Sword & Shield TCG sets
https://www.pokeguardian.com/384075_what-does-the-mysterious-d-symbol-mean-on-our-upcoming-sword-shield-tcg-cards
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