Normally when you create a course session, the entire content from the course is available for duration of the session. But in case you need to restrict the access to some modules or topics based on the date, then you can fine tune the schedule of each module within a session.
This is how the normal picture looks like in a course session that has four modules.
By assigning different start and end dates to modules/topics within course session, you can make them available on specific dates and end on specific dates, to make the picture look like this (each module is available for few days and then next module becomes available). You need to enable "Enforce end date of module/topic" setting in course session, to make module's access end on specified end date. Without having this setting enabled, only the start dates are enforced.
Or you can also apply scheduling on some modules while keep the other modules available throughout the course session duration. Like the picture below shows one module available throughout.
The above scenario was relevant for course sessions that are of finite duration, where all learner's access begins and ends on same date. But sometimes you may have a perpetual course session, that has a far in future end date. In these sessions, we typically use a Max Subscription Days property, to limit number of days for which a learner will have access to the session, after they enroll. In this case since learners access begin and end on different dates, the module scheduling with fixed dates won't work because that relies on same start/end date for each module whereas in this case even start date for each learner is different so you can't pre-determine star/end date for modules also.
To solve this scenario, course session's have a setting called "Allow learner's access to module/topic based on their start dates", by enabling that you can preset the module start/end dates relative to the course session's begin date. When the learners enroll, system will make the module's start / end dates for each learner, relative to their enrollment begin date.