A session is to be understood as an actual event: It represents the event where (usually) several participants meet to practice together on the simulator. The session therefore has a fixed date (start and end), a content (one or more courses) and a group of participants.
The session ultimately brings in revenue for the organizer and is therefore also the central billing point. Therefore, a few special rules apply to sessions, which may also affect the billing. Therefore, please read the session paragraphs carefully.