What knockdowns are untechable?

Is there any universal rule to what knockdowns are techable what knockdowns aren’t? If not, which ones are untechable in third strike and in SF4?

After a grab and after an Ultra.

For SF4 it’s sweeps, throws, supers, ultras and moves that knock you down in place like Akuma’s demon flip palm.

That includes command throws.

Wow that’s a lot of untechable stuff. What remains that is techable?

Any form of uppercut I think and Akuma’s red fireballs

Does this also apply to third strike?

The best way to research untechable knockdowns for your character is to hit up training mode, set the dummy to ‘Quick Stand’ and bust out your moves. When the dummy doesn’t instantly get back to its feet, you know that the knockdown can’t be teched.