As the name implies, symptom holders hold certain symptoms. If a body has, say, psychotic symptoms, the symptoms may be held within that alter, meaning that alter will experience psychotic symptoms more often than other alters and will likely come out during psychosis. Another sort of symptom that can be held is depression. If an alter holds depression they themself may feel depressed, the body, unless medicated, will still feel depressed, but that alter will be holding the vast majority of it, meaning they will experience it the most. This sort of thing will happen with any sort of symptom the system sees as needing to be stored.