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The purpose of User Acceptance Testing (UAT) is to test use cases, that is specific day-to-day operations in the life of an end-user and their required tasks to perform their job function
UAT Testing differs from System Testing in that System Testing is focused on software functionality. UAT Testing is focused on job function, that is day to day operations can be performed. Therefor access to your ITSM system is not needed to build UAT Test cases, in fact it is discouraged to use HEAT as your focus should be on building use cases about day to day operations, operating procedures, such as Logging an Incident, searching for customers, assigning 2nd level support, and so on.
Regression testing refers to testing of existing functionality to ensure that recent changes do not adversely affect existing features. For example, when implementing Asset Management, the existing Incident functionality is tested to test use cases such as creating an incident, creating a task, updating incidents, closing tasks, incidents, etc. In addition to UAT Test Scripts, you will want to do a comparison of existing workspaces, existing functionality, and existing features, to ensure existing functionality has not been impacted.