The unit tools test all SAP development work that handles business object processing for the connector. Also, the unit test tools enable you to test the interaction of your work with the ABAP components of the connector. The test tools allow you to test your development work as an online user (real-time) only.
The differences between testing the connector as an online user or as a background user are described as follows:
The connector runs as a background user, so it processes in a single memory space that is never implicitly refreshed until the connector is stopped and then restarted (therefore it is critical in business object development to clear memory after processing is complete). Since you are an online user, memory is typically refreshed after each transaction you execute.
For more information, see Developing business objects for the ABAP Extension module. Any problems that may occur because of this (for example, return codes never being initialized) are not detected using the test tool; only testing with the connector will reveal these issues.
SAP's transaction code may present different screens to online users versus background users (usually for usability versus performance). The test tool only operates as an online user. The connector only operates as a background user. Despite this difference, unit testing should get through most of the testing situations.