What rules are you using? NIRSA? There's an official's manual in the back of the rulebook. Four official's used are R, LJ, FJ, BJ. A U isn't really necessary as contact at the line can be watched by the R and LJ. Book covers positioning for goal line, scrimmage kicks, and other plays. I think you can order the book at
http://www.nirsa.org, costs about $7.