The term "Implementation layer" is used as opposed to a "Framework layer". It is in this context that the term "implementation" is used. The framework layer is used to store reusable rules that can be shared across multiple implementations, whereas an implementation layer only contains rules specific to that implementation.

In the past, Pega tends to be more specific about framework (FW-) vs implementation layers. However, an implementation layer can easily behave like a framework layer when a new implementation layer is built-on the existing implementation layer. Hence, there is less emphasis on creating framework layers. In fact, the wizards in Pega portals simply creates rules in the implementation layer.

Since you want to create a new data class leveraging an existing class, therefore you simply use the class in the implementation layer.