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by Sidnicious 3522 days ago
I was going to say that it signals launchd's process group — every process spawned by launchd, which always runs with PID 1. However, the `kill` manpage confirms your hunch:

  -1      If superuser, broadcast the signal to all processes; otherwise
          broadcast to all processes belonging to the user.
(This is on macOS/iOS, Linux might have slightly different semantics.)