A Side Deck is a pile of up to 15 Cards, separate from the Main Deck and Extra Deck. The Side Deck is used in Matches between Duels to change the contents of the Main Deck and/or Extra Deck. You don't have to have a Side Deck, but it can assist you in Duels against specific opponents.
Before the Match, if using a Side Deck, show it to your opponent without revealing its contents. Just to let your opponent know whether you use a Side Deck and the number of Cards in it.
If you are using sleeves for your Side Deck, they must match with the sleeve used for your Main Deck, whether if the Card is going to be sided into the Main or Extra Deck.
The Side Deck can only be used during Matches, and only in the Matches' second or later Duels.
When you use a Side Deck to alter the content of your Main Deck and Extra Deck, the number of Cards in each Deck must remain the same.
At the end of the Match, any Cards that have been in the Side Deck must be returned to the Side Deck.