Its sort of a US second division. USL and MLS aren't linked at all through promotion and relegation or anything like that, but its the level below MLS.
The USL has been around for a while. One day maybe they could join and have a regulation system. But I dont see it happening for a long time. Soccer needs to grow in popularity here in the states first.
Promotion and Relegation isn't going to happen and its not going to happen. Its not American and owners have way too much invested to see that happen. Also, USL cannot afford it in terms of money on contracts or facilities.
Kind of hard to, when it has to compete with American Football(especially College), Baseball,Basketball(especially college) and even Hockey.