Your Filofax is whatever you need it to be. If you just want a notebook with movable and replaceable pages, it can be that. If you want a flexible diary, with different page layouts for different purposes, it can be that. If you need a lot of reference notes and diagrams with you all the time, it can be that too. It can be all of those things, if that's what you need, or anything else.
With a range of sizes available, you can have a small Pocket Filofax in your pocket, or a big A5 Filofax in your bag - or both! It's the classic answer to the need for a more flexible type of notebook, and all the things that made it a great organisation tool in the 1920s, and the choice of the upwardly-mobile in the 80s, still make it the perfect answer now for those who still appreciate the analogue ways of organising and taking notes.