Knurling is a crosshatch pattern that’s machined into the bar while it turns on a lathe. Here is a Wikipedia page that describes it: Knurling.

Traditionally, barbells have knurling at the grip points. Simply put, it makes it easier to hold onto the bar. Most barbells have knurling in the center, even though one-handed barbell lifts are uncommon in modern times. This is a vestige of past weight lifting practices, since one-handed lifts were much more common a century (or more) ago.

You’re right that there’s not much clear, unbiased information about barbells on the ‘web. That’s because there are very few true standards to which writers can refer. Each sports sanctioning organization uses different equipment; they all have different standards and practices. For instance, the barbells used by powerlifters are different than those used by the weight lifters in the Olympics (though superficially, they resemble each other).

Since most hardcore weightlifting and powerlifting is a club sport rather than a widespread pastime, it seems information is guarded by members and passed down by word of mouth, so as to avoid helping out their competitors.

Thanks for the comment, I wish I could provide more info.

Weightlifters need to get their hands pretty far apart during the snatch. A 7-foot bar is a necessity for big guys who snatch.

But most people who work out at home don’t snatch or do other overhead lifts other than the military press or some variant thereof. They can get away with using a shorter bar, and some people like shorter bars because they don’t require as much workout space.