You use stacks in Studies to group together your notes, the same way you use folders to group files in Finder. Your Library can contain many stacks, each containing tens or even hundreds of notes.

There are two main types of stack: built-in stacks like All Notes and Due for Study which update automatically, and stacks you make and maintain yourself. These stacks are not only containers for your notes, they also determine how the notes get scheduled for study, and the default settings for new notes you add to the stack.