[launchd-dev] Launchd "System" session type

The launchd man page for OS X 10.8 describes five session contexts: Aqua, LoginWindow, Background, StandardIO and System. The "System" context appears to be new since OS X 10.7. It is not described in TechNote 2038 and I could not find more about it in any of the launchd man pages. If it were to be described in that TechNote's "Table 1: Types of launchd agent," how might the "System" session type be explained? -- Jeremy