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You can manage & track your card collection and decks once you create an account and login.
For casual play, you are allowed to use whatever parameters your play group has decided on. For competitive play, in Limited, you are only allowed to draw "outside the game" cards from your Limited pool.
Editor's Note: In our initial ruling, we misstated our answer and accidentally ruled that "outside the game" cards must be limited to one's sideboard. This is incorrect.They later changed competitive rules to require battle cards to come from the sideboard.