This is one of those weird little details that pretty much doesn't matter, because you just try both, and you do that every time, and it's fine. Maybe knowing the difference saves you 15 seconds.

I had an aha moment a little while ago when I realized "justify-content" works like the "justify" buttons in Google Docs/other word processors -- left/center/right. It's not a perfect parallel, since justify-content changes its meaning if the flex-direction changes, but it matches with the default behavior.