The spec currently requires at least two dots in a hostname string for the mongodb+srv scheme (e.g. "foo.example.com" but not "example.com" or "example"). This requirement is mentioned in the spec, but we don't explain why. Since this isn't a general requirement for SRV records (see: RFC2782), we would do well to explain this design choice in the Justifications section.

david.golden's comment may serve as a basis for this explanation:

IIRC, the number of dots requirement was a security-related issue, to try to guarantee subdomains. So that "good.com" SRV record can't return "db1.evil.com".