A subject close to my heart, on which I have posted before...
TOPC, have the previous replies answered your question? Because I understood you to say "When" are they used, not what is the definition.
Do you mean, for example, MSA within 25nm of the A/D and beyond that...what? One of the others - if so, which? How to choose which of the available terrain clearance methods you will use?
I, too, would like to see the answer to that one.