Immunes took up the trades and skills of engineers, field medics, carpenters, and craftsmen. However, they were also fully trained and would be expected to fight in the infantry or cavalry if necessary. The discentes were probably exempted, like the immunes, from standard combat and camp duties since their main occupation would have been time-consuming technical training.
Some known classes include:
Discens Architecti - Trainee engineer or artillerist.
Discens Phalangarium - A soldier training as a phalangarius during the 3rd century AD several experiments were conducted with Macedonian phalanx tactics, apparently requiring special training in weapons handling and drill.