References

ref      = [alias ":"] path ["#" fragment]
alias    = [a-z0-9][a-z0-9-]*          ; declared in docref.toml
path     = repo-relative POSIX path, no leading "./", no spaces
fragment = symbol | "@" region-name

Examples:

src/lib/server/markdown.ts#applyFootnotes      same repo, symbol
src/api/handler.go#Server.VerifySignature      same repo, method
src/lib/server/markdown.ts#@footnote-ordering  same repo, region
open-secret:src/api/handler.go#VerifySignature cross repo, symbol
config/default.toml                            same repo, whole file

No fragment means the whole file is the anchor.
No alias means the containing repository; the ref resolves against the working tree.
An alias must be declared in docref.toml (section 6); an undeclared alias is a configuration error.
Line numbers and line ranges are deliberately not part of the grammar. They rot on the next edit and would undermine the whole design.