All user behavior in Brainscape is centered around a Class. A Class is the "container" for all your Decks of flashcards, as well as the main unit of sharing and permissions.
In other words, you can only invite other users to a Class, and adjust their permissions for accessing the flashcards in that Class. You can't share a single Deck with a user outside of a Class.
This distinction is one of Brainscape's important differentiators that encourages users to create more organized content, with a more holistic degree of collaboration, than other study platforms.
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