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

Note:

• Category X residues cannot be discharged overboard. On completion of discharge, the tank is washed and the tank washings pumped ashore.
• Category Y residues remaining in the tank after stripping in accordance with the P&A Manual can be discharged to the sea while tank cleaning provided that the conditions stipulated by MARPOL annex II are met.
• Category Z residues remaining in the tank after stripping in accordance with the P&A Manual canbe discharged to the sea while tank cleaning, provided that the conditions stipulated by MARPOL annex II are met.