What’s the difference between passing false and true to ‘resolvingAgainstBaseURL’ when initialize a NSURLComponents instance?

It makes only a difference if you create the URL components from an NSURL which
was created relative to another NSURL:

let baseURL = NSURL(string: "http://server/foo/")!
let url = NSURL(string: "bar/file.html", relativeToURL: baseURL)!
print(url.absoluteString)
// "http://server/foo/bar/file.html"

With resolvingAgainstBaseURL == false, the URL components
represent only the relative part of the URL:

let comp1 = NSURLComponents(URL: url, resolvingAgainstBaseURL: false)!
print(comp1.string!)
// "bar/file.html"

With resolvingAgainstBaseURL == true, the URL components
represent the fully resolved URL:

let comp2 = NSURLComponents(URL: url, resolvingAgainstBaseURL: true)!
print(comp2.string!)
// "http://server/foo/bar/file.html"