@Naerling - ... It is not logic, it is not abstract and it is not UI...
But it's an option/indicator for an object or operation. For example DialogBoxs' types should be added to the same assembly where DialogBox class are defined. Again when DialogBox result is returned it will be a DialogBoxResult i.e. Ok/Cancel etc.
So i would also prefer to put all enums in Entities assembly i.e. interfaces & class definition assembly.
Thanks,
hemant