This is a separate Deck of cards you can use to change your Deck
during a Match. After each Duel in a Match, you can swap any card
from your Side Deck with a card from your Deck and/or Extra Deck to
customize your strategy against your opponent. The number of cards in
your Side Decks must not exceed 15. The number of cards in your Side
Deck before and after you swap any cards must be exactly the same
luật trong rulebook là after each duel, là sau mỗi trận trong match mới được sử dụng side deck, còn trước đó thì phải sử dụng main deck.
và side deck là để thay các card 
