(Sorry for my bad English.)
I like Johnny.Decimal: To determine the sort order by numbers. In addition, it seems to me that the limitation to 10 areas of 10 categories of 100 topics make sense in most cases (even not to wide nor to deep). If necessary, deviations are possible and I have a lot of them as you see below. Less useful for me, however, is the J.D project extension PRO.AC.ID. So I switched to AC.ID-PRO. As a small business owner, I have structured my system based on J.D, PARA and GTD as follows:
11 - 19 Projects [and other support material according to GTD]
11 Clients
11.AB1 [acronym to get more than 10 client-IDs, sorted by name]
11.AB1-000 Administration
11.AB1-001 [up to 219 Projects with no realation to year]
11.AB1-22A Accounting 2022 [repeating every year ≥2022]
11.AB1-22P Payroll 2022 [repeating every year ≥2022] …
11.AB1-22T Tax 2022 [repeating every year ≥2022]
11.AB1-22V VAT 2022 [repeating every year ≥2022]
12 Other Projects [business & private]
12.22001 [ID up to 999 items per year]
13 Next Actions
13.22001 [ID up to 999 items per year]
14 Waiting for
14.22001 [ID up to 999 items per year]
15 Somedy, maybe
15.22001 [ID up to 999 items per year]
21 - 29 Business areas of responsibility
21 Management
21.01 Businessplan …
31 - 39 Private areas of responsibility
31 Finances
31.01 Bank Account …
41 - 49 Resources
41 A - Z
41.01 A - C
41.02 D - E
…
41.08 W - Z
42 AC-Topic 1
42.01 Theme-ID 1
43 AC-Topic 2
43.01 Theme-ID 2 …
[51 - 79 for possible special resources]
81 - 89 Archive
81 Projects
81.01 Clients
81.02 Other Projects
82 Business area of responsibility …
91 - 99 Backup
91 Smartphone …