Mathematical Classifications
Mathematicians use a scheme called the Mathematics Subject Classification, (MSC) which includes a “how to use“, as well as a long history of being revised to reflect changes in the field, and I would guess, practice in how to effectively classify things.
It has a General and Miscellaneous Topics section, too.
Articles must be given a primary classification, and may be given arbitrary additional classifications. The first article in the first volume I was published in was 54C40, 14E20 secondary 46E25, 20C20.
That’s (54C40 Algebraic properties of function spaces), (14E20 Birational Geometry:Coverings), (46E25 Rings and algebras of continuous, differentiable or analytic functions {For Banach function algebras, see 46J10, 46J15})*, 20C20 Modular representations and characters).
Google doesn’t seem to be specialized in searching these things. Those 4 numbers as a search don’t return the specific paper, but then, the specific paper isn’t online. There are search engines [link to http://wgate.bibsys.no/search/gen?lang=E no longer works] that are able to search by MSC. (It’s under “Class”) in that link, or try to navigate in Norwegian [link to http://wgate.bibsys.no/search/gen?lang=N no longer works]. I did, before finding the English link.
UPDATE: The * after the {see 46J10, 46J15} was going to be a footnote, explaining that {braces} represent prioritization–you must check to see if 46J10 or 46J15 are better fits.