Protocol
ai.txt
Also: ai txt, AI usage policy file
Formats vary between proposals. Most use simple key and value lines stating permissions and a contact address.
The KailxLabs ai.txt at https://www.kailxlabs.co/ai.txt is an example. Treat the file as a public statement of intent. If a crawler must be blocked or allowed, do it in robots.txt and in any firewall or CDN bot settings.