Difference between System and Global Daemons
No Difference

Once running, there is no difference between the locations. The origin of the launchd job determines the authorisation needed to manage the job.

Both /Library and /System launchd jobs can run as any user. Per-user jobs are limited to their owner's user and group.

Editing of the /System launchd jobs is restricted by System Integrity Protection.

From man 5 launchd.plist:


  
  ~/Library/LaunchAgents         Per-user agents provided by the user.
  /Library/LaunchAgents          Per-user agents provided by the administrator.
  /Library/LaunchDaemons         System-wide daemons provided by the administrator.
  /System/Library/LaunchAgents   Per-user agents provided by OS X.
  /System/Library/LaunchDaemons  System-wide daemons provided by OS X.