Euro 6 alphabet and emission zones?

[DickP]

Hi,

I know euro 6 is the current compliance requirement for many emission zones, but there are also 6a 6b 6c and now I think 6d versions of euro 6? Does anyone know whether the emission zone requirements will likely soon focus on the alphabet specifics of euro 6 or remain blanket euro 6 till / if euro 7 materialises?

Thanks,