The main 2 commands are: All Lights On and All Units Off, thus preventing accidentally turning on an empty coffee pot.
X10 does have a All Lights Off command that is not directly supported by AHP, but is supported by the SDK.
Not all light units respond to it.
I use multiple BVC computers to send All Lights off and respond via listeners to shut down various monitors in the house at bedtime.
As far as I can tell, my older WS467's do not respond to the All Lights Off command.