Do generalist journals try to make sure every subfield is adequately represented? If so, how? I am curious: what steps do generalist journals follow to try to ensure that every subfield (e.g., number theory, differential geometry, combinatorics, etc. for pure mathematics) is adequately represented, as the journal deems suitable?
To ask the question in a different way: a generalist journal will typically publish many more papers in one subfield compared to another subfield (in an absolute sense and/or relative to total volume of papers in that field). How does this &quot;publication rate by subfield&quot; come to be? My personal interest is in mathematics, though the question applies to any academic field.
There are two sub-questions here: one for high-volume, &quot;good&quot; journals (e.g. Proceedings or Transactions of the American Mathematical Society) and one for low-volume, &quot;top&quot; journals (e.g. Annals of Mathematics, Acta Mathematica). It is a given that such a journal would have a diverse editorial board (in some cases, subject to constraints such as university affiliation), which is already a partial answer to the question. So the question is whether journals conscientiously or systematically do more than this.
My understanding is that for a journal like Proceedings or Transactions, each subject editor has a certain page quota of articles that they can recommend for publication. What I don't know is whether such a page quota is the same for each editor, or whether this varies by subfield. Or how they decide how many editors per subject. Presumably one subfield may be significantly larger than another, or a single editor might end up being the contact point for various subfields. I also don't know how final decisions are made based on the recommendations of subject editors.
For the very top journals, the discretion of the editors would presumably be a more important factor. But is &quot;we've already published in this field recently&quot; or &quot;we haven't published in this field recently&quot; often a consideration?
I think you are suggesting that there might be some sort of quota system and I doubt that such a thing is done, or even viable. You are interested in fairly broad areas of specialization within mathematics and, while I'm not in the game anymore, I have to guess that there are plenty of papers submitted in each of the categories (or they would be dropped or coalesced).
But the editors, and especially the editor in chief, is most interested in publishing the highest quality papers. Those that are interesting. Those that make serious advances. Those that suggest new ways of approaching mathematical questions. The &quot;balance&quot; will come naturally, due to the sheer number of papers.
But, if I'm the editor in chief (as I'm not), and I have two papers, one in a recently popular field and one in a less popular one, I'm going to choose the better paper. The other consideration would be far down my list of criteria. If I have a great paper and an ok but not earth shattering one, it is an easy choice.
And if there is some imbalance between the number of papers published in a subfield, I'll have to guess that it is related to the number of practitioners and the current state of the art in that field. Some fields have fewer researchers, and some threads of research get &quot;stuck&quot; for a time. Publishing &quot;extra&quot; papers in that field seems counterproductive at the moment decisions need to be made.

For the specific technical questions about quotas and such, you should ask the EiC of TransAMS, for example.