10-19 Administration

When I start a new project at work, the first thing I do is create a 10-19 Administration category.
Note that this only applies in certain scenarios. I don't do this for my home projects.
I've found the following categories within that category to be repeatedly useful. Standardising them across all projects helps the brain to memorise them. (I'm typing this page from memory – no reference to any systems.)

11 Miscellaneous

Be very careful with this category. It is necessary but it should contain almost nothing. If you put something in here you're admitting that you don't know what it is.
I just started a project and the first note I created was 11.11 What we know so far. Things have happened in this project before I arrived, and I want everyone just to dump their brains in to this note. I'll then take the contents and organise them properly.

12 People & organisations

Projects involve people. You need to recruit them, get them access to systems, get them security clearances, whatever.
There are scenarios that I haven't documented yet where I think you should use an alphabetical sorting within a JD category of People, but this isn't one of them. The things in here likely follow the same pattern as all of your other JD items: specific tasks that occur sequentially in time. As a result, they work well with sequentially numbered IDs.
10-19 Administration
12 People & Organisations
12.01 Alan's onboarding pack
12.02 Sue's security clearance
12.03 Microsoft engineer one-week contract

13 Organisations

Up to you if you use this or merge it in to 12 People. Depends on the project, otherwise just keep it spare and use it for something else.
Note that if I don't use it I would not move 14 up to 13. That would break my mental model. Just leave 13 spare, who cares.

13 Scope & requirements

If it's a simple project I'll use this to keep the proposal documentation and any other documentation relating to the requirements.
It's always convenient to be able to come straight back to the 'source of truth' when you're deep in a project and someone asks you to do something that you know isn't in scope...

14 Timesheets & expenses

Either yours or those of the people you manage. See exceptions to the rules on the main site for more information on how I use this.
I'll sometimes call this one 14 Schedule & time, which fits a bit better with the whole project management language.

15 Finance & money

"Does exactly what it says on the tin."

16 Travel

Anything to do with travel is always at 16. No reason, always has been, just is.

17 Meetings

It's sometimes useful to have a place to stash meeting minutes.
Depending on the complexity of the project, the number of meetings, and their importance (are you referring back to them in a contractual context?), you might want a 'meetings' category somewhere more prominent.
For example, a large data centre migration I managed had an area 60-69 Vendors and subcontractors. Within there each vendor was a category, and within there I had IDs for the various weekly status meetings.

If you're a PMP®

For projects managed with a formal framework – I'm familiar with the PMI's PMP, but just adapt this to your needs – you can either adapt the system above or create a dedicated area just for this. For example:
20-29 PMP
21 Scope management
22 Schedule management
23 Cost management
24 Quality management
25 Resource management
26 Communications management
27 Risk management
28 Procurement management
29 Stakeholder management
If you are a PMP you'll notice that I missed integration management. Items therein tend to fit easily in another section. Of course you could always use 20 for this if you wanted.