The "Lost Deck" (ロストデッキ Rosuto Dekki) is a different deck from your main deck which replaces a player's main deck after the use of the "Belost" ability of the "Lost World" flag.
A Lost Deck consists of exactly 30 Lost Cards (cards with the black backing at the right) and only Lost Cards (main deck cards cannot exist there). Lost Cards can only be put into a Lost Deck, and cannot be put into the main deck due to their different backing.
When changing from the main deck to the “Lost Deck”, the normal cards in areas other than the deck area (i.e. the field, soul, hand, gauge and drop zone) remain as they are, and the fight continues with the Lost Deck placed in the deck zone as a new deck. For example, in the case of a "put a card from your deck into your hand" ability, when resolving this ability, one card is added from the current deck to your hand.
In areas other than the deck zone, it is possible that normal cards and Lost Cards are mixed together. For example, there may be a case where 3 normal cards and 2 Lost Cards are in your hand. For effects such as “drop one of your opponent’s gauge” or “choose a card from your opponent’s hand at random”, when choosing from a hidden zone where normal cards and Lost Cards are mixed, the cards must be chosen in a situation where it is possible to tell normal cards and Lost Cards apart.
It is not possible to mix normal cards and Lost Cards in the deck zone. For example, if an effect like "Overturn Ice Emperor, Miserea" or "Gate of Pardon, -Forgiven-" would return cards that originated from the main deck into the Lost Deck, main deck cards will not return and will remain there. Only Lost Cards will return to the deck. Another example, after placing the Lost Deck on the deck zone, and you use an ability such as “return a card from your gauge to your deck”, a normal card cannot be put into the current Lost Deck. If there are Lost Cards in the gauge, choose a Lost Card from the gauge and return it to the deck. If the gauge only contains normal cards, no card can be returned to the deck.
The Lost Deck may only contain a maximum of 4 cards with the same card name (excluding the buddy monster). Even if a card has a different ability or illustration, the maximum of 4 copies is kept. If a card effect states that "This card is also regarded as X", only the name printed in the name box will count towards the "max 4 copies" rule (See: Super Lethal Formation/Rulings). Both cards will have the same name, and are viable targets for any card that may support them.
You may only choose a Lost World monster as your buddy, only if you have the Lost World flag in your main deck. If you have the monster as your buddy, you do not flip over and reveal your buddy at the start of a fight. It is only flipped and revealed when your flag becomes the "Lost World" flag. During a fight, if the buddy is face down, it is not possible to buddy call, and the face down card in the buddy zone will be regarded as a blank card with no information.
If not otherwise specified, you cannot prepare multiple Lost Decks, and switch a different one for each match during the same tournament. When using card sleeves, it is necessary to use different sleeves for both your main deck and Lost Deck in order to tell them apart. When preparing for the game, the fighter using a "Lost Deck" must place it in such a way that it is visible to the opponent. It is highly recommended to place it on the right of the drop zone.
Tournament officials and judges may decide on the placement area of the Lost Deck, if it is deemed that there is insufficient space for it on the fight table. With acknowledgment from both fighters, a judge may rule for them to place the Lost Deck into a container such as deck cases or deck boxes, but care must be taken not to mix them up with other cards unrelated to the fight at hand. A fighter who wishes to use a Lost Deck must shuffle both the main deck as well as the Lost Deck during the preparation part before a game. After which, it will no longer be possible to check the contents of the Lost deck which has been removed from game.
The main deck removed from game, via the "Belost" ability, is removed from the fight with all its cards face down and shall become a hidden zone. Both fighters will not be able to check its contents. During the fight, cards from the main deck that was removed from the game cannot be referenced, and cannot move to another area. Also, it is not possible to move any cards from any area to the main deck, which was removed from the game.
To be added.
For purposes of differentiating, Bushiroad's staff has emphasized in certain terms like "White Deck" (白きデッキ Shiroki Dekki) and "Black Deck" (黒きデッキ Kuroki Dekki).