SULE I stands for Small Unit Leadership Exercise I. It is a fireteam-sized exercise where you conduct attacks against an enemy, or complete a humanitarian mission. These can be fun but also frustrating. Just like FEX (Fireteam Exercise), you'll be given a combat order, which you will then pick out the relevant bits and issue your own order to your fireteam. Then you will conduct the exercise and hopefully defeat the enemy and get a passing score.
Having good knowledge of the Combat Order will help you with this exercise. I've made a completely separate folder for the combat order!
It's especially important to not let yourself get defeated if the exercise doesn't go according to plan. Just be confident and move onto the next portion.