the -10 series is for the GI that operates the equipment.
the -20 series is for the unit maintenance people that do basic repairs.
the -30 series is for the major facilities that rebuild things from the ground up.
the -50 series is for the manufacturers
The second digit is for the next higher user so, for instance, a -14 manual covers everything from operator to theater of operation level repairs, while a -35 manual covers everything from a divisional repair/rebuild facility all the way back to the manufacturers.