When Canon describes the 1Dx as having "100 to 51200 native ISO (expandable to ISO 50-204800)" what does that really mean? Does "native" mean setting the sensitivity of the sensor itself and "expandable" mean setting it after the signal comes off of the sensor? I've googled around and haven't seen a great explanation.
In the sensor systems we are most familiar with, the electrical outputs of the individual photodetectors are amplified (on an analog basis) and then given to an analog-to-digital converter (ADC) to be digitized. Both amplifiers and ADCs are "multiplexed"; that is, there is not normally one per photodetector. There are many schemes for this multiplexing.
The basic mechanism for changing the ISO sensitivity of the system is to change the gain of the analog amplifiers, normally among a fixed set of predetermined values (controlled by a digital command to all the amplifiers).
The resulting repertoire of sensitivities are often described as the "native" sensitivities of the system.
However, ISO sensitivities greater than that given for by the greatest provided amplifier gain - as well as sensitivities between those given by established amplifier gain values, or lower than those, - are provided by digital scaling of the digital values out of the ADC.
Sometimes the former of these - or all of these - are described as "expanded" sensitivities.
This technical article from 2006 gives some insight into this concept of sensor operation:http://dougkerr.net/Pumpkin/#CMOS-APS