sorry, I just figured out which lesson you are referring to: Best Practices for Designing a Data Model.
Understanding flow and business data has nothing to do with Pega in particular. You should be able to define your data needs on a whiteboard - and not think in terms of any app development platform.
The concept of defining data model artifacts as applying to ...-Work- classes and ...-Data- classes is unique to Pega. Another app development platform will have a very different way of defining and organizing those same data model artifacts.
Draw the process on a whiteboard. Then, at each step, define what data is needed to transact each step. Taken together, this data model defines your flow data.
Next, consider those steps that require human interaction. What data does each actor need to complete their task? That is your business data. In general, business data can be defined as a subset of flow data.
For example, re your last question: Can we call the customer's data (received through Data classes) as business data? The answer is yes - but it is also very likely that some of the customer's attributes will be flow data as well. When you shop online, your CustomerAcountNumber is used to manage, process and track your order. This meets the flow data definition because this processing and tracking is most likely automated. However, if you call the customer service number, your CustomerAccountNumber may be presented to the call center agent so they know who you are. This meets the business data definition.
flow data = all of the data elements needed to transact (complete) the process
business data = the data elements needed to provide context for a given human-oriented task; typically a sub-set of the flow data.