Strategy Card
Strategy Cards are given away with Duelist Packs 1 through 9 and Duelist Packs Yugi and Kaiba They give advice on older cards and their ability to play alongside, or together with the cards showcased in the Duelist Pack it's released in. Each Duelist Pack series has 10 Strategy Cards, 9 of those being actual Strategy Cards, the 10th being a Card Checklist.
See List of Strategy Cards for a complete list of them.