In most states in Australia agents can have Exclusive Agency agreements with Sellers which allows only that agency to handle the sale of the property

You can enter the listing agreement expiry date or use the set to button to choose from 30, 60, 90 or 120 days from the Listed Date or set your own date. 

It is purely an internal setting so you can switch on a Notification to alert the Consultant to re-sign the agency agreement with the seller when the selling authority is about to expire.

The expiry date wont affect your listing's presence on websites if it expires. It is purely an internal setting as a reminder for the agency or sales agent