Area B traffic enforcement area is based only on poluttion/emission levels of the vehicle. There is no subscription/vignette available for this traffic zone, unlike the
Area C for which the entry can be purchased.
What you have to do
In order to enter the zone (which is the largest in Europe) through its ANPR gates, you have to make sure that your vehicle is compliant with
Euro X emission standards. In particular, LPG and electric vehicles are free to enter, and the X in the
Euro X standard varies on the fuel type (gasoline vs diesel) and will likely increase in the future, in a restrictive way.
The emission standard is displayed in the vechicle registration card (Zulassungsbescheinigung, example Euro 4).
What they have to do
They have to match the license number of the car with the emission levels to issue a traffic ticket. The City Of Milan has online access to vehicle registration data in Italy only.
Foreigners are immune
This is verified (source)
Foreigners (almost) not punishable
A primary issue is with vehicles with a foreign license number. Cameras can take pictures of them, but the control system cannot determine whether they can drive.
In fact, their environmental class is unknown to Italian authorities: one must get real-time access to foreign databases where these cars are licensed, which is impossible. The only thing one can do is to rely on traditional spot checks by the police (when they have time and personnel and if on-board documentation is sufficiently clear).
Basically, if you drive a foreign vehicle the city's Metropolitan Police is unable to obtain data in real time and must let you go.