Evidence hooks provide extension points where customized business logic can be added to an evidence processing. For example when removing evidence, the evidence controller calls an evidence hook where extended functionality can been added.
The registrar process works in conjunction with evidence hooks. Its purpose is to provide the ability to customize business logic on a per product basis. Each product can register with an evidence sub-pattern its own hook. Once a product is registered, the evidence controller will enact the extended processing for the process specific to that product.