The M in MVC means model, but what does a model really mean? The simplest definition is that model refers to the application problem-domain entities and their behavior, where behavior can be implemented in the entities themselves, or in a dedicated service class.
Classes that contain only properties with no logic are called Plain Old CLR Objects (POCO) classes. These are the type of classes that work best when received as input or returned as output from your APIs. It will save you many headaches if you stick to POCO classes in the external surface of your APIs. This will also keep you from making mistakes, namely exposing sensitive information such as your database structure, as your APIs grow. On the internals of your APIs, you can use any type of classes that you like, and in Chapter 5, Persisting Data with Entity Framework, you will learn how to use the Entity Framework (EF) core to work with classes that map to database tables and allow you to store data and query it.