Until now, we used test cases as the pivotal point in writing self-checking behavior specifications of components. But they can serve other purposes, such as collecting particular test classes to run them as a group too, since pluggable processors allow execution adjustments to highly diverse demands. This chapter will start by explaining the architecture behind this mechanism and advance, for a profound understanding, with what it takes to write a custom extension. With these insights, we'll be ready to discuss a couple of valuable use cases and also draw attention to some tradeoffs. Since one of the most important applications is parameterized tests, we will cover the available approaches thoroughly in the last section. The topics we are going to look at briefly include: