irok,
if you are selected as a S/O on the 777 fleet, do you fly the -200, -300 and the -300er or do you only fly the one variant?
And likewise with the 747 fleet, do you fly all variants including cargo?
holdmetight is correct.
My understanding is that you fly on any B777-operated route that requires an S/O to legally extend the maximum flight duty time. Given that, you will probably spend most of your time on the -300ER but may also fly the -200 and -300 on occasions. I have seen -400 S/Os fly on cargo routes