Definition
Crawlers that fetch web content, e.g. GPTBot, PerplexityBot, ClaudeBot. Allowing them is a prerequisite for being cited.
Why it matters
Being fetchable is the precondition for everything else. A site that blocks the retrieval bots cannot be cited at answer time no matter how good its content is.
How to measure it
Test with the real user agent, not just by reading robots.txt. A permissive robots.txt in front of a CDN rule or a firewall that returns 403 to those agents is a block in practice.