The autocreate parameter is part of the ConnectPolicy key in the server configuration file, server.cfg:

ConnectPolicy autocreate=1,autoconnect=1,automigrate=1,desktop=0,dialog=0

and by default it's set to '1' in order to automatically createa new virtual desktop when the session type is pre-defined in the player configuration.
 
When autocreate is disabled (autocreate=0), the list of all the available desktops, physical and virtual ones, will be instead displayed to the user. 

Note that with this configuration, the user will always have to click the New desktop button in the list of running remote desktops, even if the session type is pre-configured in his/her connection file.