In a nutshell, data model creates placeholder for a object attributes (properties) whereas class structure (inheritance) is used as an aid in reuseability and maintenance ie) Things common between a truck and car object are place in a vehicle object. The vehicle object is inherited by both car and truck
There are several sites on the internet which explain these basic programming concepts such as:
Let's define the terms clearly. Enterprise Class Structure - there is enough material on Pega website is about organizing your Work and Data across Organization, Division, Framework and Implementation layers, and regarding Pattern Inheritance and Direct Inheritance,
My pet peeve with PDN is lack of material about how to do Data Modeling in general (or any references to it), and then specifically how to actually define Data Classes across the various layers of the Enterprise Class Structure.
So my 2 cents, if one wants to understand Data Modeling, then do it outside of Pega. Then, once you understand Enterprise Class Structure for Data, you can apply knowledge you gained regarding Data Modeling to correctly define your Data Classes in appropriate layers and with appropriate inheritance (hint if you are using Pattern Inheritance for Data classes, something is VERY wrong).