An answer of November 19, 2007, states:
If the scrimmage appears as part of the regular assignment process and is listed by the league assignor, it should be considered by the referee to be officially sanctioned. The teams did not call the referees directly to make the arrangements, but went through the official assignment procedure with the league assignor.

My question: What if the referee for the scrimmage was not assigned by the league assignor, is the referee covered by USSF insurance. In my case the coach went out and obtained the center referee. I was not aware of the assignment but the referee was paid by the league coach

USSF answer (September 17, 2010):

Even if both teams are affiliated with US Youth Soccer, the game itself would not appear to be affiliated if it does not occur within the framework of an affiliated organization. If the teams did not go through the official procedure, we suggest that the referee check with local refereeing authorities to avoid possible problems with liability and insurance coverage.