To exempt vents from the application of Subsections 4.3.4. and 4.3.5. which would otherwise impose certain vent requirements, if certain measures are taken [vents conform with good engineering practice].
This is to limit the probability that excessive internal tank vacuum or pressure will develop in normal or fire conditions, which could lead to the failure of the tank, which could lead to the escape of liquid, which could lead to harm to the public.