Pega Collections is a suite made up of Customer Service for Financial Services and Pega Marketing for Financial Services (CDH). You are welcome to implement only 1 application but most of the out-of-the-box functionality to support Collections will not work.
I cannot list out which features are application specific in the context of Collections since they are shared features. For example, The collections case is maintained in the Service application and the contact strategy and orchestration is maintained in the Marketing application. Together they deliver the Collections Contact Strategy feature.
I would be more than happy to discuss any specific questions you may have if you contact me directly. Kind regards,