What are Categories or types of Noxious Liquid substance or NLS as per MARPOL annex II ?

Category X

NLS which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a major hazard to either marine resources or human health and which therefore justify the prohibition of the discharge into the marine environment;

Category Y

NLS which, if discharged into the sea from tank cleaning or ballasting operations, are deemed to present a hazard to either marine resources or human health or cause harm to amenities or other legitimate uses of the sea and therefore justify a limitation on the quality and quantity of the discharge into the marine environment;