Regular souls are from butchering, they cannot be stockpiled (though withered souls will be put in the refuse stockpilie); put a temple near your butchers (or vice versa).
Megabeast souls come from megabeast corpses that get dissected in the slaughterhouse. This results in various materials and the megabeast soul and trophy. The soul is used in the megabeast kennels (along with the trophy) to ressurrect it as friendly, or to give a dwarf 10 to all combat skills (also in the megabeast kennels). The kennels must be researched (field of Titanic Beasts).
Titans and forgotten beasts are not megabeasts, so you don't always get a corpse usable in this way when a major invader appears.
.
As for the thread, it's recommended to put dwarf fort questions in the
[Dwarf] Gameplay Questions thread. Not sure if this is the thread you were thinking before.