In Mathematics, what is the standing of the journal Proc. AMS? This is a question specific to mathematics.


  
  what is the status of the journal Proceedings of the American Mathematical Society (Proc. AMS)?
  


It seems to be a generalist journal that only publishes short papers, and has a massive output (in 2018: 12 issues on 5485 pages). The blurb says :


  "To be published in the Proceedings, a paper must be correct, new, and
  significant. Further, it must be well written and of interest to a
  substantial number of mathematicians. Piecemeal results, such as an
  inconclusive step toward an unproved major theorem or a minor
  variation on a known result, are in general not acceptable for
  publication".


Yet on Scimago it is ranked below most of the journals I have heard about (in their math/stats ranking it is #256, edit: in the subranking that lists mostly pure math journals, it is still lower than most I know edit ends). Nevertheless, I see that very well established researchers sometimes publish there (I won't give names obviously).

So to be more specific :


  
  do you always submit somewhere else first, and it the paper is rejected then consider Proc. AMS ?
  or maybe is publishing there an act of "giving back to the community by promoting the AMS" ?
  or is it mostly a journal for anybody who needs a quick publication (as the workflow seems quite fast) ?
Scimago is a clearly false, possibly semi-fake ranking. It is very easy to check this: for instance when I clicked on your link I got the following amusing result:

Vital and health statistics. Series 2, Data evaluation and methods research   is considered of higher ranking for mathematicians (!) than the Annals of Mathematics.

There is no need to elaborate more.