Although the sticker was first introduced in 3 large cities, it has been rolled out to cover whole geographical areas or departments as can be seen here.
https://www.crit-air.fr/en/inf…zones.html
This has implications, albeit likely to be exceptional, in terms of driving from say Atlantic Spain (San Sebastion for example) to Cherbourg. Your route may cross 4 Crit air zones. It remains to be seen how this might be policed, but for the sake of €5 it seems prudent to be as prepared as possible.
One further piece of information, if you try and move or remove the sticker, it is designed to come away in an unusable state and apparently the word VOID appears on the remains of the sticker.
Davy