Ref. 1:
Stage II Vapor Recovery
Stage II vapor recovery systems are used to control releases of gasoline vapors when vehicles are refueled. A specially designed nozzle is used at the gas pumps to collect vapors from the vehicle's gas tank and return these collected vapors to the service station underground storage tank.

Ref. 2:
Two basic types of Stage II Vapor Recovery Systems are available to the site owner or operator: the balance system and the assist system.

The balance system operates by creating a sealed loop between a vehicle's fuel tank and the site's storage tank. This process requires a specialized nozzle which both dispenses fuel and collects vapor. An outer cover or bellows (often called a "boot") surrounds the nozzle and forms a seal with the opening to the vehicle's tank. As gasoline is pumped into the vehicle, fumes are displaced and flow into the bellows. The fumes are then collected into the storage tank.

Or if your customer wants UK English rather than US English:
"Petrol vapour recovery process".

Gaspendelung am Tankwagen
Vapour Recovery at the tank truck
Seit einigen Jahren wird beim Befüllen von Tanks mit Otto- und Dieselkraftstoffen das Gaspendelverfahren angewendet. Die auf dem Markt erhältlichen Schlauchleitungssysteme wiesen wesentliche Nachteile auf. Ein speziell für die Sicherheit und Bedürfnisse der Tankwagenfahrer zugeschnittener Gaspendelschlauch sowie eine neu entwickelte Mutterkupplung räumen diese Mankos zukünftig aus. Hierbei spielt der Aspekt Sicherheit die größte Rolle. Für die Standardisierung dieser Komponenten bildet die Normung die Basis für den Erfolg in der Praxis.

For some years when loading tanks with Petrol and Diesel fuels the vapour recovery process is used. The hose assembly systems available on the market have some basic disadvantages indicated. A vapour recovery hose which is particularly adapted to the safety and the needs of the tank truck drivers as well as a new developed female coupling should be the solutions for the future. Here the aspect security plays the biggest role. For the standardisation of these components the standardisation groups are the basis for the success in the practice.

http://www.oldenbourg.de/vulkan-verlag/rou-rrr0108.htm