The Session Expiry Date is the date at which an "unbooked" Session automatically expires or "disappears" from a Teacher's Available Sessions list. It is automatically defined when the Teacher sets his "Session Expiry Term".

This can be one of the following:
2 hours,
4 hours,
12 hours,
24 hours, 
48 hours.

Case.
Suppose a Teacher chooses to set their "Session Expiry Term" to 48 hours. Now all his Available Sessions (Sessions available for booking) will now expire 48 hours before their Start Date and Time. This means that if now is Sunday noon, their next Available Session can only be on Tuesday at noon or later and all Sessions that were available between now (Sunday at noon) and Tuesday at noon have now automatically disappeared or expired and therefore can no longer be booked. This gives a certain freedom to the Teacher.

The Session Expiry Term is also important for "rescheduling" a previously booked Session.

Case. 
Suppose that today is Sunday noon and a Student has booked a Session for Wednesday at noon with this Teacher. However, something has come up for her and she wishes to reschedule.
IF she still has "Rescheduling Quotas" left, she can reschedule but must do so before ... and here it comes ... the "Session Expiry Date".
IF she does, she will be able to book that Session again in the future before her Course ends.
IF she does it AFTER this date but BEFORE the Session Start Date and Time, it will be considered a "cancellation" and she will lose this Session.
IF she does nothing and does not show up, she's a no-show and the Session is considered a cancellation.