Organisation, classification, taxonomies, hierarchies, and catalogues are complex concepts.

I've run across the Johnny Decimal concept a few times, and though it's interesting, and probably better than no organisation, I'm not sold on it. That's strongly informed by a long deep dive into other classification and bibliographic schemes.

One key distinction is between a location-based classification system and an indexing or tagging system.

A large bibliographic institution with an extensive history which maintains its own versions of both concepts is the US Library of Congress.

The Library of Congress Classification is based on a scheme originally inherited from Thomas Jefferson for organising his own personal library whose donation to the US Congress inaugurated the Library of Congress. It is based on 21 top-level divisions, based on letters of the Roman alphabet from A-Z, excluding the letters I, O, W, & X). These are further broken into a vast number of further divisions, with the ultimate aim of identifying an individual work AND locating that work within the library's physical archive. Books, as physical objects, occupy a distinct space, and one space only, which cannot be shared with another book.

The Library of Congress (LoC) Subject Headings are a set of descriptions of works. Unlike the classification, a book may be assigned multiple subject headings, corresponding to its major themes. These headings are not used to define a book's location but rather aid in its discovery when searching by subject.

Both of these classifications have evolved over well over a century, are the subject of ongoing efforts and development, and have had to address the fact of change in both the underlying subject matter (e.g., the Austro-Hungarian Empire, Soviet Union, and Colonial Africa are no longer extant) and how subject matters are addressed and referenced (psychology, sexuality, race, economics, scientific discoveries, and technological advances have all made old terms obsolete or deprecated, and introduced new ones).

These are not the only classification and cataloguing schemes used by the US government, let alone other bibliography-manangement institutions elsewhere. The Superintendent of Documentation (SUDOC) system is used by the Government Printing Office to manage its publications and archives, and is organised principally around government offices (Cabinet divisions amongst the Executive, others in the Legislative and Judiciary), as well as date.

Some libraries don't organise works by subject but instead assign a location code to works independent of subject by which the works may be requested by patrons. (These are typically "closed-stack" systems.)

There are other classifications: Dewey Decimal (superficially resembling the LoC classification), Decimal Classification, and Colon Classification are amongst the most widely used.

Digital storage has different requirements than physical. Documents needn't have a single storage location or address. They may be part of numerous distinct systems, often with idiosyncratic or incompatible management or organisation themselves. Documents are far more likely to be live, with ongoing development and adaptation as opposed to the static state of a print archive. "Documents" may in fact be data held in databases, wikis, or other systems, and be associated with multiple projects.

One generally useful set of concepts for organising documents is the Dublin Core Metadata Element Set (DCMES) with a set of 15 defined elements: Contributor, Coverage, Creator, Date, Description, Format, Identifier, Language, Publisher, Relation, Rights, Source, Subject, Title, and Type. A document might have zero or more of each element associated with it (e.g., multiple authors, translators, illustrators, and/or editors as Contributors). And it's been observed that some elements are themselves ambiguous (e.g., what is the difference between a "contributor" and a "creator"). But in general this at least provides a useful framework for considering works.

<https://en.wikipedia.org/wiki/Dublin_Core>

In looking at document management I've found a few other elements are useful to consider:

- Project: the task being worked on.

- Workflows: what is being done with documents.

- Document lifecycle: Often consisting of a set of stages: creation, edits/updates, acquisition, cataloguing, format conversions or transformations, transmission or publishing, and deacquisition or destruction.

- Security and/or distribution scope. Keep in mind that metadata itself has significant value and should be considered withing your data management / privacy / security policies.

- Teams and participants. Both within and external to your organisation.

- Organisational evolution. Personnel, departments, divisions, enterprises, and governments come, go, and change. The terms used now might not apply in a year, or 10, or 100. (Your document lifescycle might not extend to 10 or 100 years. Then again, it might.)

Most usually, I'll find I use the following in organisation:

- Date. Generally the start date of a project. Organising documents by time makes a great deal of sense, and creates natural search spans: day, week, month, quarter, year, decade, etc.

- Author/owner. The person, department, or institution which created or "owns" the document. Within a sufficiently large organisation, these should live within a departmental / divisional structure.

- Title. How the document is generally referred to. This can be startlingly ambiguous, especially internally. Third-party documents tend to have far more fixed and established titles, though even these can be tricky. E.g., "The White Album" is not actually titled "The White Album".

- Other relevant distinguishing information.

Where possible, I'll try to store information under a single filesystem directory hierarchy, though in practice things tend to live in multiple places: a shared code repository (public or private), a wiki or other platform, databases and application data. If possible these are tied together in some useful fashion, though any such organisation tends to be approximate and lagging reality at best.

With a sufficiently large organisation, a librarian role who manages overall storage and resolves disputes is helpful. Though you'll be exceedingly lucky to have someone actually assigned to such a role.

Johnny Decimal is interesting, and may well work for some cases. I doubt it meets all needs, and certainly isn't the One True Path for all individuals and organisations.