I think in the US, you must affiliate with one of the USSF affiliated leagues if you intend to play (inter-play?) soccer with any of the USSF affiliated leagues. I think some of it has to do with insurance requirements. In my limited understanding, there are four major "youth" associations leagues can choose to affiliate with. OK, I just looked at Google and these are the ones listed. I found one league, SAY Soccer, that one proponent listed as being the "best league for youth soccer" but it seems like a small league/association. Youth level U.S. Soccer Development Academy Super-20 League Super Y-League United States Youth Soccer Association US Club Soccer American Youth Soccer Organization In my area, with a 20 mile radius, we have 3 AYSO regions, 32 USYSA clubs, and 2 US Club leagues. I believe we have some PDL and other other similar clubs. I looks like some of these leagues listed on Google are part of the USL family. It is my understanding is that a league\team\club can affiliate with multiple leagues/associations if they intend to play leagues inter-play or tournaments. It all seems rather confusing and political. Since I only work with AYSO games, how does this affect the referees or do they have their own set of politics that cross over the top of the leagues/clubs/levels.
As an AYSO ref, if you want to become a USSF ref, you can cross certify. IIRC, you must be at least an Intermediate AYSO ref first. AYSO Intermediate or Advanced can (once in a lifetime) become an 8; an AYSO National can become a 7. Once you cross-certify, you must then meet all USSF ongoing requirements. (Cross-certification goes the other direction, too -- and AYSO requires AYSO certification to do games [though I have a vague recollection there may be a FIFA-badge exception].) AYSO is the only entity in the US authorized by USSF to separately train and certify soccer referees. All other affiliated games are refereed by USSF refs.