[2] Similarly, a session denotes a collection of one or more process groups.
The distribution of signals to process groups forms the basis of job control employed by shell programs.
The TTY device driver incorporates a notion of a foreground process group, to which it sends signals generated by keyboard interrupts, notably SIGINT ("interrupt", Control+C), SIGTSTP ("terminal stop", Control+Z), and SIGQUIT ("quit", Control+\).
Graphical user interfaces, such as where the X display manager is employed, use a different mechanism for implementing login sessions.
POSIX prohibits the change of the process group ID of a session leader.