The corpus callosom is a bundle/band of nerve fibers found deep inside the brain. This connects the right and left hemisphere. Corpus Callosotomy or otherwise known as "Split-Brain" surgery is a procedure where the corpus callosum is cut. This is done to control epilepsy but only as a last resort.
The surgeon can partially or completely cut the corpus callosom. If a partial dissection is enough to control seizures, they would stop there. However, if the seizures goes uncontrolled, it is completely dissected.