Corporate fiscal calendars are, IMO, best thought of as essentially made up. They start and end at arbitrary-ish¹ points in the year, and thus their numbering does not always match the "calendar year" (the one you're normally familiar with).
The fiscal year is a pretty standard finance concept, so anything in that area isn't going to bother with an explainer; you're just expected to know.
From the outside looking in, it just seems nuts. If I ever found a company I'm going to do my best to get my fiscal year to == calendar, if I can. It also annoys me when companies needlessly use it internally. Mine does, I have no idea when the quarter ends anymore.
For example, there are easy to read sites with explanations like https://www.thebalancemoney.com/how-do-i-determine-my-compan..., and less easy to read things like Form 1128.
The fiscal year is a pretty standard finance concept, so anything in that area isn't going to bother with an explainer; you're just expected to know.
From the outside looking in, it just seems nuts. If I ever found a company I'm going to do my best to get my fiscal year to == calendar, if I can. It also annoys me when companies needlessly use it internally. Mine does, I have no idea when the quarter ends anymore.
¹ish. I think the IRS has a few rules.