There are two scenarios when the Expense Sheet can be reopened:
1) The person who approved the Expense Sheet or the Expense Sheet Approver can "Reopen" approved expense sheets when changes are required.
2) The Owner of the Expense Sheet can 'Reopen' a submitted Expense Sheet, which pauses the approval process for an expense sheet that has not yet been approved by the Expense Approver or a direct manager.
The 'Reopen' command changes the state of the Expense Sheet back to draft.
You can find more information here: http://usermanual.clarizen.com/#6-1-expense-sheets-overview.
All the best,