Category Z in Annex 2 refers to Noxious Liquid Substances (NLS) that, 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. Consequently, they justify less stringent restrictions regarding the quality and quantity of the discharge into the marine environment.
In simpler terms, these substances are considered less harmful than those in Category X or Y and thus, discharge regulations are relaxed accordingly.
