LeaderProcess is a sandbox setting in Sandboxie Ini. It specifies names of programs that are considered primary in the sandbox, and when they stop running, all other programs in the sandbox are stopped as well. For example:
. . . [DefaultBox] LeaderProcess=iexplore.exe
iexplore.exe is Internet Explorer.
Related Sandboxie Control setting: Sandbox Settings -> Program Stop -> Leader Programs
See also: Program Settings.