To add closure code and close a release, go to Releases and click the release from the list.
Click Review from the sidebar.
The Closure stage has 5 tabs, namely Details, Tasks, Notes, Approvals (if configured in the associated workflow), and Status Comments.
Under Details, select the closure code from the drop-down and provide a short description of the release closure. Using closure code, you can provide details on whether the release is closed due to cancellation, rejection, completion, or other reasons. (The SDAdmin can define release closure codes under Setup >> Customization >> Release Management >> Closure Code.)