If the fly.php is called without the variable $act defined, then the process start a new NO-SOD session.
To do so, here are some actions executed:
Instantiate a new session object, and try to start it. If session started with the no-sod version, then:
In the case the session didn't started, the error returned will be "SIZ" => Sizing error, no more free session slots, try again later.