I would start with reading article 338 and then look through article 690 for any specific rules for USE and PV.
Personal, I don't use direct buried cable simply because I have seen it fail many times and being that I am likely going to be the one to fix it I would pipe it. BTW, a friend of mine ran a 600' run of use-2 for a solar run and fortunately, he added a few extra conductors because some of them failed in less than a year. One rock is all it takes