The picture above shows the ACU Gold sticker on the back of a Gmac motorcycle helmet
The Autocycle Union GB (ACU for short) is the body responsible for the governing of motorcycle racing in the UK (the UK equivalent of the AMA, American Motorcyclist Association, in the USA).
Originally, there were 2 standards for ACU approved helmets, silver and gold (ACU Gold being the higher standard). The ACU Silver approval has since been discontinued.
The ACU Gold sticker has been recognized by many worldwide motorcycle racing bodies as a high-level helmet safety standard for motorcycle racing & many of those organizations will only allow motorcycle racers to use helmets with the ACU Gold sticker.
The ACU Gold sticker isn’t required for a helmet to be road legal in the UK (a helmet has to have either the BS 6658:1985 or ECE22.05 standard to be road legal). But it is a slightly higher safety standard and generally indicates a slightly better quality helmet.