By default, user programs operate in the TS scheduling class. Kernel routines execute in the sys scheduling class.
Processes inherit their scheduling class from their creators.
Scheduling classes can be specified at the user, group or process levels. It is not possible change the scheduling class for a particular thread. The scheduling class for a thread depends entirely on the scheduling class of the process that the thread belongs to. Since scheduling classes are inherited, and user shells are by default of scheduling class TS, it is necessary to make a system call to convert a user process to the RT scheduling class.