There are five types of sessions which you can join: Practice, Qualify, Time Trial, Race and Test. Sessions are separate; a practice session does not transition to a qualifying session, then to a race. Each session must be joined and completed separately.
Once you register for a session by clicking on one of the red times in the schedule on the Session page, a countdown timer will appear at the top right of the page showing the time remaining before the start of that session. Pressing the WITHDRAW button will un-register you from the session with no penalty.
Important Note: For Race sessions, registration closes about two minutes before the session starts, at which point if you withdraw you will score zero championship points for the race, hurting your race week points average.