Topic notebooks are the most common type of notebook you will write. You will have one of them going per identifiable project you are working on. This can easily or exceed half a dozen notebooks. A large pile of tiny notebooks allows you to:
Each topic notebook is labeled with a name (and optionally, a category). As your notes extend beyond the borders of a single notebook, you'll make more notebooks with the same name, labeling them as later volumes.
That's up to you.
Anything where you are focusing your attention to a specific goal, concept, event, or idea could be a project. If you intend to pursue research or development towards something, it is almost certainly a project. If you feel it would be helpful to write a checklist about something specific (as separated from a "stuff to do today" page - a checklist joined by conceptual similarity, rather than chronological similarity), but then discover that any item on the checklist deserves a note of its own or your checklist exceeds a page, you probably have a project on your hands. Start a notebook series!
The reason you need a large pile of notebooks to use this system is to avoid the fear of running out of notebooks. Notebooks must not be perceived as a limited resource. The Tiny Notebooks system works best when you are willing to start a new Topic Notebook when you think you might need one, even if that leaves you with a few notebooks that got three pages in and then never touched again.
When starting a new topic, think of a name to describe it by. It can be as clever or blandly functional as you like. You will also need to abbreviate that name to something short, since you need a version of that name you can use to refer to pages in that notebook from other notebooks, if it comes up.
The front of each Topic Notebook is labeled with the notebook's full name, its volume number, and its category. This lets you identify your notebooks and it will make them easy to sort later.
Just like in your Daily Carry, add page numbers and a Table of Contents. These will be features of every notebook you will create in the Tiny Notebooks system.
Usually, you will want to start your notebook series with an "Overview" explaining what the series of notebooks is about - notes on what you're doing that made you think of something as a project in the first place. If your notebook series spun off from a different series because you discovered it had a subproject in it, the first note is a good place to mention it, especially if notes on that subject are already in that earlier series.
Notes in Topic Notebooks are formatted the same way as all other notes - check out the Writing Notes page for more information.
Sooner or later, and probably sooner, you will have a project that exceeds a single notebook in length. When you run out of space in your notebook, create a new notebook the same way you created the original one, except it should be labeled "Volume 2" or higher - whatever the next volume in your series would be! Continue your notes there. Each volume has its own Table of Contents, but it can pick up right where the last volume left off - you don't need a new topic overview.
Once you have filled up a notebook or completed a project so you expect not to write in its notebooks anymore (or at least for a while), it's time to archive it.