What Is It?
Note storage refers to how the Mac note taking app stores your notes. The two classifications are database (DB) and multiple documents (MD). With database storage, the note taking app will generally present your notes in a single window and allow you to create categories (and possibly subcategories) in which to store your notes. With multi-document storage, the note taking app lets you group and store your notes in one or more documents, and will display each notes document in its own window.
What Else Should I Know?
Multi-document apps far outshine database apps when it comes to keeping high-level groups of notes separate. While it’s true that you can group notes under top-level categories like “Personal” and “Work” within a database notes app, in reality this strategy doesn’t provide enough separation. Imagine sitting down with a client who accidentally sees a notes category for a competitor, for whom you also work. Or a classmate venturing into your Personal category while you’re away from your computer. Or your private notes being forced to sync to your work computer just because you use the same note taking app at home and at work. With a multi-document notes app, you can create Personal, Work (even on a per-client basis), and other note documents that enforce note boundaries by more than just names.
With database apps it can also be tricky transferring your notes between computers. That’s because the notes database is generally hidden from view in the Finder, and not intended for you to manipulate directly.