This directory contains any non-essential binaries used exclusively by the system administrator. System administration programs that are required for system repair, system recovery, mounting /usr, or other essential functions should be placed in /sbin instead.
Typically, /usr/sbin contains networking daemons, any non-essential administration tools, and binaries for non-critical server programs.
These server programs are used when entering the System V states known as "run level 2" (multi-user state) and "run level 3" (networked state) or the BSD state known as "multi-user mode". At this point the system is making services available to users (e.g., printer support) and to other hosts (e.g., NFS exports).
Locally installed system administration programs should be placed in /usr/local/sbin.