Info about px4fmu and px4io module in pixhawk

I am working on pixhawk. I have got few doubt about px4fmu and px4io.

  1. Can px4fmu or px4io independently drive a quad? if yes than what is the benefit we get when we use both px4fmu and px4io in pixhawk?
  2. Where can we set whether to choose px4fmu or px4io?
  3. We build the firmware on pixhawk by command make px4fmu-v2_default . Does this firmware runs on
    both px4fmu and px4io?
  4. How the memory segmentation is done for px4fmu and px4io ?
  5. If both px4fmu and px4io are running on pixhawk what are there individual functionalities?
  6. If we want to add our own custom module for any functionality than which board will be responsible to process it?
    Information about px4fmu and px4io will be very much helpful for me.

Px4fmu and px4io are bit older hardware, I think nobody is selling them today.

Px4fmu is core of autopilot, here are main features and you are able to fly copter with only fmu unit, if you dont need large number of outputs.

Px4io is expansion board for better acces to connetors, to be able to connect more esc and servos, with some more features of failsafe mechanisms, if pf4fmu freeze or another failure happend.

Pixhawk is px4fmu plus px4io in one box.

More info you should found on older pixhak pages

Thank you for providing useful information.
As you mentioned px4io is just expansion board with failsafe mechanism and my understanding is in failsafe on the basis of user setting either quad try to land or it goes to home position so to do the operation of landing sensor data and access to flight stack is required my question is can px4io access the sensor data and flight stack?
And one more question is in pixhawk manual it is mentioned that it has 2mb flash memory so this 2mb is shared memory between pxufmu and px4io (this question is same as point no 3 in my original post)?

PX4IO unfortunately dint have ability(IO isnt connected to IMU sensors) to fly copter, it is last stage of fail-safe so for copter stop motors and possibly deploy parachute, for fix-wing you can regain full manual control with RC.

In pixhawk 2MB shut be memory on FMU part, but some version of this chip was faulty from manufacture so it has only 1MB