I have two children with EHCPs and have been a teacher for 20+ years so I do understand in mainstream how outcomes/success/progress is measured, although this can be very problematic as does not take account of the young person eg whose parent died a year before their GCSEs. However you could argue with broad strokes it is reasonably successful.
My youngest child will certainly never take any qualifications. At 9, and at special school, his targets are to dress himself, start trying to use the toilet, increase the half dozen words he randomly says.
A good outcome for me is that he is happy, making some progress, able to access the world.
Obviously other children with EHCPs would have completely different targets.
What do you suggest for measuring outcomes for children with profound/severe disabilities?