The best timing to write unit test case is during development. Although each code line is a potential to be a defect, not each code line should be written as tests case. Selectively choose the line/s which can cause severe defects.

Subsystem test case –

Subsystem test cases are being used by both developers and testers. These test cases are necessary to perform, but some organizations are skipping directly to system test due to effort constraints. The test cases of subsystem test are focusing in the correctness of applications or outputs such as GUI windows, invoices, web sites, generation of files etc. The cases are checking the end results of the software. There are two main important tasks of subsystem cases:

1. Testing that back-end and front-end of the same functionality are working correctly.

2. Testing that two integrated front-end applications or two integrated back-end applications are working correctly.

Example for task number one, the set of subsystem test case activities will be the following: Testing the retrieval of the correct data of parameters in window application or in web site application which defined dynamically in tables or even hard-coded in the code lines.

Window or web site = front-end

Dynamic tables or hard-coded code lines = back-end

I can agree that in some extent system test is also performing end software output tests and integrated applications tests, but the difference is huge, see both below and system test article.