Leaving the scope off of the field means "package-private". "package-private" means that any class in the same package has access. We probably want to restrict the field scope to just subclasses. Note that the "protected" scope would allow a subclass outside of the package to access the field, which is not possible with "package-private". In our case, though, we intend for the class to be subclassed and for that field to be accessed by the subclass, so "protected" is more appropriate here.
Leaving the scope off of the field means "package-private". "package-private" means that any class in the same package has access. We probably want to restrict the field scope to just subclasses. Note that the "protected" scope would allow a subclass outside of the package to access the field, which is not possible with "package-private". In our case, though, we intend for the class to be subclassed and for that field to be accessed by the subclass, so "protected" is more appropriate here.