NASL is the official 2nd division according to the US Soccer Federation so that makes USL is the 3rd division. Though it doesn't matter too much since there is no promotion/relegation.
USL teams are allowed to compete in the Lamar Hunt Open Cup but NASL teams are not allowed this year something about scheduling.
Honestly, there isn't a huge difference if any. A lot of the clubs in the NASL used to be part of the USL but broke away and formed the NASL.