For instance if we know that the team has test the Data Layer well, but the GUI layer has been harder to Unit Test then the testers should focus more effort on the GUI layer.

Sometimes applications are written with a fine-grained API layer, requiring the client to make multiple calls to the API layer for a single logical unit of work (LUW).

As the name suggests, this strategy is used when the application's client layer must make one or more calls to the API layer to complete a single transactional unit of work.