jails What is a usecase for exec.system_user as mentioned in jail(8)?

[AndAlooongCameFreeBSD]

Hi all,

I was wondering about what exactly "current user" and "system environment" means when using exec.system_user. jail(8) says:

     exec.jail_user
             The user to run commands as, when running in the jail
             environment.  The default is to run the commands as the current
             user.

     exec.system_jail_user
             This boolean option looks for the exec.jail_user in the system
             passwd(5) file, instead of in the jail's file.

     exec.system_user
             The user to run commands as, when running in the system
             environment.  The default is to run the commands as the current
             user.

Why or how would you be able to do anything in the system environment from within the jail? Could someone explain how exec.system_user and the other two (if they matter) are intended to be used?

Thanks