The input impedance can be configured to be 50 or 75 Ohms.
The units can be changed only globally to be dBm, dBmv, dBµV, V or W. The tracking generator output level is referenced into a 50 Ohms load and is displayed in the selected units. You can always chose a different unit when you enter a level with the digital keypad but this is automatically translated into the "global" preset unit.
Please be aware that the DSAs currently shipped only permit a 20 dBm adjustment range for the TG output level which equals a 1:10 voltage range. The attenuator in the device (probably an IDT F1912 or a Peregrine PE4312) should permit a 31.5dBm range but Rigol probably needs some "headroom" above and below the available 20dBm range in order to calibrate the output level.
Btw, I hunted down a few of the active components in the input branch of the DSA (those with the identification etched away) and it appears that most of them allow a pretty good margin for a higher frequency range. At least the 3.2GHz appear not to be unreasonable with the standard 815 setup. And as it seems, the bare PCB is the same for all the 800 models...
Cheers,
Thomas