Their priority rules are:

1. Design tests based on user scenarios that span system components, not on the components themselves. In today's trendy terminology, Petschenik is saying you should base tests on use cases, not product architecture. This rule is based on the observation that "developers will tend to check individual components more carefully than they check how these components are supposed to work together." System testing should fill in the gaps of developer testing, not replicate it.

   In my experience, this is a rule commonly broken by system test organizations. It is too common for testers to test features in isolation, often simply working through the reference manual (which is organized alphabetically or by feature group). For example, one tester might test the editor. Another might test the printing code. No one will discover that the product crashes when you edit, print, use the editor to revise some pages, then print again - something that millions of users will do.

2. Retesting old features is more important than testing new features. This rule is justified by two observations. The first: "The old capabilities [...] are of more immediate importance to our users than the new capabilities." Existing users already depend on old features; they can't depend on the new ones yet, because they don't have them. The second observation is that developers test whether their new code now works; they're much worse at testing whether the old code still works.

3. "Testing typical situations is more important than testing boundary cases." There are two justifications for this rule. First, developers do an adequate job of testing boundary cases. Second, boundary cases are uncommon in normal use. A failure in a boundary case will be seen by few users; a failure in typical use will affect many users (by definition).