The objective of each level is to focus on specific PDTs (potential defect types). And to do this it requires specific types of tests to be executed. The picture below shows the relationship between levels, PDTs and Test types. Note that PDTs are numbered only and the exact PDT is not listed.
This enables one to be clear as to the focus at each level and therefore focus on specific test cases for each type of test. Also this partitioning of PDTs at each level enables a clear partition of work between the various stakeholders in the SDLC