If you search Google for "Xcode build settings" the only result on the first page from Apple.com is this useless article that gives an extremely high level description of what a build setting is and hasn't been updated since May 2011.
I follow a lot of iOS developers on Twitter and I stumbled upon a tweet that had a link to this Xcode build settings reference page which seems to have detailed descriptions of every single Xcode build setting. I have never found a reference guide as complete as this. If you have ever found yourself confused by an esoteric Xcode build setting you can probably find it on this page.
One of the bizarre things about this page is that it is not hosted on developer.apple.com but is under help.apple.com and the URL is completely nondescript. Combine this with the fact that it has the
<meta name="robots" content="noindex" /> tag in its source and it looks like someone really went out of their way to hide this page.
I knew I was going to lose this URL if I didn't store it somewhere and I thought what better place to put it than has a hotlink on my blog. Google will actually search this and, who knows, a couple of iOS or macOS developers may find it useful. Oh one more funny thing about this page, the find functionality doesn't work on Safari. It will show you how many matches it found but you can't actually jump to them. Compare that to Google Chrome where it works perfectly. I have no idea what sort of red headed stepchild this page is but I am ecstatic it exists.