A field declared on the parent that only one subclass reads or writes; the parent's stored state includes a slot that does not apply to most instances the agent reasons about.
The field lives on the subclass that uses it; the parent's storage declaration carries only the fields every instance holds, dropping the irrelevant declaration from the agent's window.
Before the refactoring
class Employee {quota; // only Salesperson uses this}
After the refactoring
class Employee {}class Salesperson extends Employee { quota; }
The parent's storage carries dead weight; serialization includes ignored fields; the agent's reasoning about the parent's shape is muddied.
If the parent occasionally consults the field for type checks, pushing it down forces awkward downcasts the agent must add and verify at every consumer.
Other subclasses no longer carry storage they never touch; queries about the parent or any sibling subclass load fewer irrelevant field declarations into the window.
Pushing down a field that callers reach through a parent-typed reference forces every read or write to downcast first; each downcast is a runtime type check the agent's generated code has to handle at every consumer.