> I've been a developer for nearly 40 years (mostly in Windows, barely dabbling in linux) and I've yet to figure out what the heck that number means, or how I might predict what the correct number is when I want to look something up myself.

https://en.wikipedia.org/wiki/Man_page#Manual_sections

You usually don't have to care unless you're writing them because it only matters to a user when there are man pages of the same name in multiple sections.