The Reimbursable checkbox can be present for users when creating an expense. With this set-up, the user can choose whether the expense is reimbursable or not.
This checkbox can be enabled/disabled in Admin > Advance Settings, under Expense Settings.
NOTE: When activating this option, please take into account you're allowing the user to decide whether an expense is reimbursable or not. Meaning, even though the user used the company credit card for example, the user can still decide to define it as reimbursable.