A good appreciation of Scope for dev testing will not only help in sharpening the clarity of ‘what-to-test’ and ‘test-for-what’ but sensitise you to the PDTs so that you prevent defects. And that is what good testing should be – sensitise you, so that you can prevent.

For example, if you knew the PDTs at L1 relate to data type/boundary (etc) issues, would you not be careful while coding to ensure good validation checks so as to prevent rather than to detect & find? A clear partition of what-to-test in terms of EUT and therefore Quality levels enables clear partitioning of (testing) work between developer and the specialised tester and ensures no duplication of work.

Note that PDTs in L1 through L3 beg to be prevented rather than detected, as these are not always not complex to be detected only via testing. Being sensitive to PDTs in L1-L3 enables you be a “clean programmer” rather than only be a “good dev tester”. And isn’t it what we all strive to become?