Can I use a Category 1 certified device in a lower-risk area like Zone 2? How does the hierarchy rule work in selections?

Yes, you absolutely can. In ATEX directives, the hierarchy rule always applies: equipment in a higher category can always be used in lower-risk zones. Category 1 devices are accepted in Zones 0, 1, and 2. You might be paying for extra protection you may not need, but it is always compliant.