The sys file describes the systems your machine knows about. An entry is introduced by the system keyword; the subsequent lines up to the next system directive detail the parameters specific to that site. Commonly, a system entry will define parameters such as the telephone number and the login chat.
Parameters before the very first system line set default values used for all systems. Usually, you will set protocol paramters and the like in the defaults section.
Below, the most prominent fields are discussed in some detail.