In a trading card game, limited is a format where you build your deck from a limited pool of cards (IE from cracking booster packs at the event). In Final Fantasy there are several ways to play limited formats, similar to other TCG's. The most common formats currently are Sealed and Draft.
Sealed is a type of limited used commonly with pre-release kits. In sealed, you open a set amount of packs, and build a deck with what you pull in order to compete against other players. Because you may have to build a deck from many different elements, in sealed, you can play Backups using CP of any colour. After that, you must use proper CP to pay for abilities or play Forwards, Monsters, and Summons.
Draft is a format where a group of players open booster packs, and select one card, before passing their pack along to the next person beside them. Once all packs are open and drafted, you build a deck with the cards you have picked. For awhile in FFTCG, each player received 4 booster packs in a draft. Recently Square Enix added draft officially to some events, and switched to a 6-pack draft. However, they are now using a 5-pack draft system for all Crystal Cups using the format this year (2019). This page does not guarantee that the format will change or stay the same in the future, it is best to contact your distributor or Square Enix directly if you need official draft rules.
In draft you can have any number of a card in your deck, and the minimum deck size is 40 with no upper limit to the number of cards in your deck. The regular rules for paying CP and playing cards stand. If a card has the multi-play symbol, you could have more than 3 of it on the field at the same time, since you are allowed to have more than 3 of a card in your deck. The only other difference in draft is that you play to 6 damage, rather than 7.
Learn to Play Limited
For the best resource on how to learn to play limited formats, check out the Final Fantasy Crystarium. One of their writers, Jeff Curran, is a very experienced limited player and consistently puts out content focused on limited game play. You can also check out this Square Enix draft guide that was used at the German Crystal Cup this year. It is outdated for now, as it is for 6-pack draft, but it does give you an idea of how to hold an official draft.