In general, all the daemons follow the following global rules. There may be exceptions depending on the specific case. Normally, one daemon will be sending commands to another daemon (specifically, the Director to the Storage daemon and the Director to the File daemon).
Currently, these negative lengths are specific to the daemon, but shortly, the range 0 to -999 will be standard daemon wide signals, while -1000 to -1999 will be for Director user, -2000 to -2999 for the File daemon, and -3000 to -3999 for the Storage daemon.
Kern Sibbald 2010-08-30