TL;DR

種類	例	absoluteURL
絶対URL	`URL(string:"scheme://example.com")`	`scheme://example.com`
相対URL	`URL(string:"path/index.html" relativeTo:absURL)`	`scheme:example.com/path/index.html`
絶対パス	`URL(fileURLWithPath:"/root/path" isDirectory:true)`	`file:///root/path/`
相対パス	`URL(fileURLWithPath:"path/test.txt`, relativeTo: absFileURL)`	`file:///root/path/test.txt`

NSURL / URL 概要

let absURL = URL(string:"http://example.com")

どちらも可

let relativeURL = URL(string:"path/index.html", relativeTo:absURL)

let relativeURL = URL(string:"/path/index.html", relativeTo:absURL)

let absFileURL = URL(fileURLWithPath:"/root/path", isDirectory:true)

let relFileURL = URL(fileURLWithPath:"path/memo.txt", relativeTo:absFileURL)