ImpressPages automatically starts the session for each visitor of the website. If you use several ImpressPages websites on the same server, use different sessionName option in config.php file to keep the sessions separated.
Sessions and forms
ImpressPages forms are secured against CSRF and basic spam attacks. This is made possible using sessions. So it is very important to keep users session alive even if the user keeps filling in a single form for 30 min. To make that happen, ImpressPages sends a ping (?pa=Core.ping) request using AJAX to refresh the session. So as long as user keeps the browser open, the session never expires as it is being constantly refreshed.