I'm not aware of a discrete on code for the 990 apart from the inputs - although I think on the 950, the "TNR" button on the SL-9000 remote may have been a purely "discrete on" command that behaved the same as pushing the power button on the front panel. (Ever since we got the 950, I've always used macros that also jumped to a specific input so I didn't do much with that on the 950.)

As you say, the discrete off is the same as the power button on the remote (which only turns the unit off) and as the front panel power button when the unit is on; a pure discrete on command (as compared to the discrete input/on commands) would be the same as the front panel power button - reverts to the input that was active when the unit was powered down.