99.9% of the time you want your IDs to be folders that then contain a bunch of files. Category 12, in your example.
Some of my folders do only include a single file. I usually don’t know this when I create it, it just turns out that way.
This thread also discusses this idea, and an exception. @Heloise has a category which is ‘sewing patterns’, and in this case one ID = one sewing pattern.
In this case she did not bother creating the folder, as she knew 100% that there would only ever be one file in it. This is definitely an edge case though, just for your information.