Question:

What is the Purpose of Expired in a Vault Document?  What is the next step after the document is expired? 

Answer:

Document expiration occurs if a document is no longer valid and is no longer going to be used in Vault. 

Typically there are no further steps after expiry.

The expiration process can be configured to send out notifications to people in roles on the document at certain points prior to expiration. These notifications ask the user to determine if the piece should be renewed. If so, the expiration date is typically changed and the process repeats again sometime in the future. 

Related Documentation:

Vault Expiration Documentation: About Document Expiration