For the purpose of regulation, Noxious Liquid Substances are divided into four categories:

• Category X: Substances which, when 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 thus justify the prohibition of discharge into the marine environment.
• Category Y: Substances which, when discharged into the sea from tank cleaning or deballasting 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 quantity and quality of discharge into the marine environment.
• Category Z: Substances which, when discharged into the sea from tank cleaning or deballasting operations, are deemed to present a minor hazard to either marine resources or human health, and thus justify less stringent restrictions on the quality and quantity of the discharge into the marine environment.
• Other Substances (OS): Substances indicated as "OS" in the pollution category column of Chapter 18 of the IBC Code, which have been evaluated and found to fall outside Categories X, Y, Z, are considered to present no harm to marine resources, human health, or other legitimate uses of the sea when discharged into the sea. These substances are not subject to any requirements of the Annex.

A "Special Area" means a sea area where, for recognized technical reasons in relation to its oceanographical and ecological condition and to the particular character of its traffic, the adoption of special mandatory methods for the prevention of sea pollution by oil/NLS/garbage is required.

As per Annex I:

• Mediterranean Sea
• Baltic Sea
• Black Sea
• Red Sea
• Gulf Area
• Gulf of Aden Area
• The Antarctic Area
• North West European Waters
• Oman Area of Arabian Sea
• Southern South African Water

As per Annex II:

• The Antarctic Sea

As per Annex IV:

• The Baltic Sea

As per Annex V:

• Mediterranean Sea
• The Black Sea
• The Baltic Sea
• The Red Sea
• The Gulf Area
• The North Sea
• The Antarctic Area
• The Wider Caribbean Region

In 1992, MARPOL was amended to make it mandatory for tankers of 5000 DWT and more, ordered after July 6, 1993, to be fitted with double hulls, or an alternative design approved by IMO (Regulation 13, Annex I, MARPOL).

The requirement for double hulls, initially applied to new tankers, was extended to existing ships under the 1995 program, stipulating that all tankers would have to be converted when they reached a certain age. This measure was adopted to be phased in over a period of years.

Although the double hull requirement was adopted in 1993, proposals for accelerating the phase-out of single hull tankers were discussed. As a result, in April 2001, an IMO resolution adopted a revised phase-out schedule for single hull tankers, which entered into force on September 1, 2003. In 2003, further revisions to the requirements were made, accelerating the phase-out schedule even further. These amendments entered into force in April 2005. A new regulation on the prevention of oil pollution from oil tankers when carrying heavy grade oil banned the carriage of this in single hull tankers of 5000 DWT and above. For ships with 600 DWT to 5000 DWT, this ban applied no later than the anniversary of their delivery date in 2008. The final phasing out date for Category 2 and 3 tankers was brought forward to 2010 from 2015.