What Spell Cards (スペルカード, superukādo) are and how they actually work is not known at this time, but they appear to play an important role in casting powerful character-specific magic. During a Spell Card trance, the user's offensive and defensive capabilities appear to increase greatly. In some cases, the user becomes completely invincible for the duration of the trance. However, the user may only cast the spell that was specified by the Spell Card until the trance ends, limiting its overall usefulness.
In the Touhou shooting games, Spell Cards are used as life-saving "bombs" by playable characters and as a source of danmaku bullet patterns by enemy characters. In Immaterial and Missing Power, Spell Cards attacks are the equivalent of "supers" in 2D fighting games (a special-condition, high-damage, high-priority attack). In Scarlet Weather Rhapsody, there are three types of cards: System (same for everybody), Skill Cards (affecting special moves of chosen character), and Spell Cards working like cards from IaMP.
Spell Cards were probably the first known instance of a shooting game's bullet patterns being given official, personalized names. Many danmaku fans appreciated this extra touch, as it clearly showed that the developers of the Touhou series cared about the patterns as much as the fans did.
It was specified in Perfect Memento in Strict Sense that the Spell Card system was developed by Reimu and first used in the Scarlet Mist incident. They were designed to give even weak youkai and humans a chance to win, as well as to provide youkai a way to defeat the Hakurei Shrine in a fair duel. They are described as pieces of paper bearing the descriptions of their respective spells, along with the possible approval of the shrine maiden.
In Silent Sinner in Blue, Marisa describes them in terms similar to a regular duel: You must dodge your opponents or be crushed by them. However, the difference between spell cards and a normal duel is that "the most beautiful one wins. It's a very mental contest." As it seems in the following, Spell Cards regulate rules for the time a duel is held such as: Used equipment, a time limit, winning and losing conditions and restrictions(mostly for the dodging part). Additional for the equipment (Like Marisa using the Mini-Hakkero while her "Final Spark" Spell Card lasts) Spell Cards however don't seem to be limited to magic. If someone for example is able to call the power of a god like Sanae or has a certain amount of Equipment like Nitori. Everything that can be used as an attack counts as a spell card.