[launchd-dev] The MachService key

In trying to understand how a process works, I found a launchd plist with a MachService key. From launchd.plist(5), I learn that that this is to "specify Mach services to be registered with the Mach bootstrap sub-system". I know a little about the Mach bootstrap sub-system, but I can't find much information about Mach services. Does anyone know a book or other resource where I could learn about Mach services? Thanks, Jerry Krinock