[dotancohen]

Those planes are allowed to be flown for specific testing purposes only. You cannot fly passengers nor even cargo with them. And each flight crew member must have a very specific, essential role. You can't fly e.g. a tourist and call him "mission specialist" to get around the rules.

[cjrp]

I'm thinking of kit/owner-built aeroplanes. They can fly like regular (type-certified) aeroplanes, but have to be placarded (e.g. https://www.aircraftspruce.com/catalog/cspages/passgrwarn.ph...) https://www.faa.gov/aircraft/air_cert/airworthiness_certific...