1. Every deck that you use to DS duel with needs to have at least 41 cards. This is needed because your DS is not an actual part of your deck.

2. Your Duel Spirit is still a card, so it is still limited to 3 cards. This means that if you have a Duel Spirit, you can only have a maximum of two other cards with the same name in your deck. These other cards are no DS's, so they won't have a DS effect.

3. Your Duel Spirit can't be affected by any cards, since it is next to you, and not actually Removed From Play. (There's just no other place to put him in DN)

4. Your Duel Spirit can be Special Summoned to your side of the Field once in the entire duel. (In any Phase) This means that your Duel Spirit can attack and defend like any other monster in your deck. Since the Duel Spirit is a very special card, summoning him is a big risk; If you choose to summon it, it also loses it's immunity against effects. This also means that it can be attacked by your opponent's monsters. When your DS (Duel Spirit) is sent to the Graveyard, or is Removed From Play after it has been summoned, you automatically lose the duel!
Note: When your DS is flipped face-down, it will not be able to use it’s effect until it’s flipped face-up again.

5. No effects can be activated in response to it's summon. This means cards like Torrential Tribute, Solemn Warning, Solemn Judgement, Bottomless Trap hole, etc can't be used.

6. When your Duel Spirit is returned to your Deck, or returned to your hand after it has been Summoned, it will return to it's original location. It can't be special summoned afterwards.

7. When your Duel Spirit is used for a Synchro, Fusion or XYZ summon it is moved back to it's original location, outside of the game, instead of the grave or rfg. Please be aware of rule 4