ECE-2205 is a specific regulation a helmet has to meet in order to be used in the UK. Helmets are put through an impact absorption test to ensure maximum safety standards.
SNELL is an American testing system usually displayed on American helmets. If your helmet has both ECE-2205 and SNELL on the back this means your helmet is ‘Dual Homologated’ which allows use both in the UK and US. The following models meet this specification – RX-7 GP, RX-7 RC (Carbon), Quantum ST and RX-7V