Go to Tutorial and then go down to information and I believe it should be listed under either Sorceress or Sorceress' and their Powers.
GameFAQs: Final Fantasy VIII (PS) Time/Ultimecia Plot FAQ by Sir Bahamut This has the data as well as some of the Ultimania translated in the "additional information" section. Just scroll past the Time theory hoopla and R=U stuff to get canonical info.
Reading it though, it is true that a person has to have a strong potential for magic to be chosen for embodiment but that is the only requirement, its not a hidden power it just needs a host who has the potential to control it which makes pretty good sense, though I feel the actual in-game part is pretty vague about this as well.