To be a symptom holder means that you hold a symptom of something. If your system and your body has depression, one member may hold those symptoms. This could mean many different things depending on the system. For some, that member would be in a constant state of depression, in others it could mean that they simply hold it but lack the symptoms affecting them, sort of like shielding the other members from feeling it when in headspace. It depends on the system of how it could work