At the highest level, an end user executes a sequence of use cases to accomplish a job. This is termed as a end-to-end business flow. At the highest level, therefore the EUT is a “Business Flow”. This is a combination of “Use cases” or “Requirements” and hence the next level of EUT is a “Use case” or “Business Requirement”. To execute an use case, we invoke a series of features of the system and therefore drilling down, the next level of EUT is a “Technical feature”. A  technical feature is implemented using some  structural components like a screen, class, task, API, service, therefore the final lowest level of EUT is a “Structural component” with a component being any one of the above mentioned.

In HBT there is a clear notion of what the EUT in each level as outlined below:

End-to-end flowL6, L7, L8, L9
Business requirement /Use caseL5
Structural component (screen, class, API, service, task)L1, L2, L3