To be a system it means to have alters or system members. A system typically forms as a form of protection for someone's mind and develops in childhood. When you're in a system, you tend to have a role or purpose in the system. These roles help a person function. When you aren't controlling the body, which is called fronting, you are able to go into a headspace, which is the inner world of a system. In some cases you simply stop existing for a bit, but in many you are able to go into this headspace.