SIGCONT
The resume ring: it wakes whatever was frozen.
The other half of the pause: it resumes a process stopped by SIGSTOP or SIGTSTP. It holds a unique privilege in the tower — it wakes the process even if the signal is blocked or ignored, because the resumption happens before any handler.
Day to day it is the shell that rings it: fg and bg send SIGCONT before handing the process back to the foreground or the background. A handler can still be hung on it — useful for redrawing whatever changed while the program slept.
$ kill -STOP $(pidof -s backup.sh)
(peak hour; the server breathes)
$ kill -CONT $(pidof -s backup.sh)
(the backup picks up where it left off)