A Top-level Class
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Inherits from @baseclass
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Inherits from Work-
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Contains most of your business logic
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Is usually the same as a “bottom level” class
A Top-level Class in Pega PRPC inherits from @baseclass, which is the ultimate parent class in the Pega class hierarchy. Work- is for work objects (cases), not top-level classes. While top-level classes contain business logic, that's not what defines them as top-level. They are not the same as bottom-level classes, which are leaf nodes in the hierarchy.
Every class in Pega ultimately inherits from the built-in @baseclass, which sits at the very top of the class hierarchy and provides the foundational behavior all classes share. A top-level class is a high-level organizational class (like an application's base class) — it typically holds little business logic itself (that lives lower down), and it is distinctly different from a 'bottom level' concrete class that actually stores instances.