Weiss Schwarz is a game in which players build 50 card decks to play against their opponents. A deck must consist of exactly 50 cards and no more or less. Decks are composed of 42 playable cards, ranging from characters to events and exactly 8 climax cards for a total of 50.
Decks can be made from any set and its affiliate sets (sister sets). This means that sets that are released within the same franchise can be used together. For example, the sets, Date a Bullet (DAL/WE33) and Date A Live (DAL/W79), can be used to build decks together. However, this may not always be the case between affiliate series. For example, Prisma Fate/Illya (PI/SE18) cannot be used with Fate (FS/S03). Similarly speaking, although shared set codes such as DAL usually signify that the series sharing the code are affiliated, that may not be true. Variants to codes can exist such as in Fate/Stay Night (FS) and Fate Zero (FZ) yet still allow them to be played together. However, this may not be true as well since Fate/Stay Night (FS) and Fate/Grand Order -Absolute Demonic Battlefront: Babylonia- (FGO) are two separate series that cannot be used together.
*This is not always the case and you should be checking official site to see which sets are allowed to work in combination with another*
As such, always check the official site to see which sets can be used together. The link is as follows: https://ws-tcg.com/rules/deck_rule/
Unless indicated otherwise, you may only have 4 copies of any card in a deck. This includes cards that share a name or have special rulings indicating so.
For instance: