To remove certain tanks from the application of Sentence 4.3.11.4.(1) and to size vent piping according to the maximum possible vapour flow if it is not possible to fill the connected tanks simultaneously, or the connected vents have a vapour recovery system.
This is to limit the probability of excessive internal tank pressure, which could lead to failure of the tanks and the escape of liquid--or blowback of liquid or vapour at the fill opening--which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.