Hi,
The Action Log is found at the bottom of the incident and problem work item forms.
For every object in the system we keep track of every property change and relationship add/remove, who made the changes, and when they were made. You can see this information on the form of pretty much every object in the History tab.
For more information, please review the link below:
Action Log, History, and Auditing in Service Manager
http://blogs.technet.com/b/servicemanager/archive/2010/02/04/action-log-history-and-auditing-in-service-manager.aspx
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