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 that are able to search by MSC. (It’s under “Class”) in that link, or try to navigate in Norwegian. 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.