Well, there is no sign up or login. The app is completely open for anyone to use. There is also no central database where everyone's writing is saved. All your data is saved to a SQLite database that lives in your browser. So users can save as much as they want. Browsers today can handle saving few gigabytes of data. That's a lot of text.

There is a server but that's only used during sync to temporarily store the encrypted data before passing it on to the destination device. It's basically a buffer. After sync completes, the data is deleted.