What is the difference between segregated and clean ballast tank in tanker?

Clean ballast refers to the ballast stored in a tank not permanently utilized for ballast (see segregated ballast below), but has been cleaned prior to use so clean ballast stored in it, if discharged from a stationary ship into calm water on a clear day will produce no evidence of oil, sludge or emulsion on or under the surface of the water or on adjoining shorelines.

Segregated ballast refers to ballast stored in tanks which are permanently utilized for the carriage of ballast, and are completely separated from the cargo oil and oil fuel system.