Personal Substitute in Workflow
By Suresh Kumar Parvathaneni, from link.
Requirement: Need to have at least one substitute who can act upon work items in your absence.
Important: The appointed substitute would be able to see all your work items. A substitute cannot be assigned for only particular area of organization or any other item. So need to ensure that the substitute have the necessary authorizations to act upon the work items.
Procedure:
1. Go to SAP Business Work Place (TCode: SBWP)
2. Now from the menu bar, select Settings à Workflow settings à Maintain Substitute
3. Highlight/Select the name on the window, in this case SAPDEV02 and click on 
“Create Substitute”.
4. List of users available in the system are displayed.
5. Select the required substitute user name. Following screen appears.
Validity field describes about the period in which the setting would be active. Note that the dates on the screen would range from the current date to 31st Dec 9999.
Regarding the checkbox “Substitution active”, it is advised to leave this checkbox unchecked for system performance reasons. A check in this box indicates that the substitution is “permanent” and if left unchecked it is treated as “as-needed”. A permanent substitute will automatically receive the primary users work items in their own box. In the case of “as-needed”, substitute must manually adopt the work items from the primary user’s inbox.
6. Save the entries
7. Click Enter to leave the screen.