The first choice is to decide how fetchmail should start. If the mail server is on the other end of an expensive telephone or ISDN connection, you may want to start it rather infrequently. In this case, you let the computer start it at intervals by the clock. Intervals in seconds must then be zero.
If the mail server is on the local network or if the computer is on a direct line to internet, fetchmail can be started frequently without extra cost. Set the interval in seconds to a reasonable value and select start at boot.
If the computer is booted frequently you may also decide to set the interval to zero and have it started at boot to get a one-time retrieval.
Or if you only want to create the configuration file or start fetchmail yourself, choose manual start.