Well, Splitting a soul means literally cutting it into three pieces. This is usually done when a soul becomes too powerful to control and may destroy a piece of existence. In all cases, however, the three pieces are very specific:
The Mind: tends to be slightly evil in actions. Usually quite skilled magically and very, VERY, sneaky. For those of you keeping track, that's Canfie.The Heart: This is usually the extremely good part of the soul. This piece of the person will lean towards lighter organizations and will actually usually side with Heaven. This is Charia, people.
The Body: Without the compassion or brains of the other two pieces, what's left? The answer that thing that looks like the original person. This piece of the soul will carry the majority of the original skills and abilities and will also look like the whole person. Neat, eh? The Body tends to be rather more neutral in conflicts, but will side with the Heart over the Mind. The last of the sister organizations is headed up by the Body of the soul of Kamille.
If all three pieces should come back together, the energy used to split them would rush into existence once again, causing a huge imbalance to occur and throwing everyone off for an extended period of time. Also, if the Mind is allowed free reign over the other two parts at merging, the person will lean towards evil tendencies and will surely cause a lot more destruction than is necessary. In short, re-merging can be bad news. However, sometimes it is necessary to re-merge to continue the growth of the entire soul as the sum of its parts. Charia, Canfie and Kamille may once again be themself, but it ain't gonna happen for a bit yet.
For those of you wondering where the names come from, the splitter usually chooses the names of the new forms. The Body is consistently given the name of the original person. In the case of Charia and Canfie, the names come from the Sumelian tongue and mean, literally, the heart and the mind.