A system to organize projects
This system lets you define your own areas and categories. It is critical to design your system such that none of the areas or categories overlap. For this system to be successful, there should be only once place anything can ever be.
Here's one example of someones system using Johnny Decimal https://notes.leahferguson.com/00+Meta/04+SOP/File+Hierarchy+SOP
I can see a system like this being useful if you frequently execute projects with a similar structure (or that would fit into a similar structure). I have not yet been able to wrap my head around how I might use this system for my personal data.