Parameters of hdr_beg -i <arg>

haproxy 2.84 (Ubuntu 22.04) Maybe someone can help me finding the documentation for the -i switch to the hdr_beg directive.

I’m seeing usages of:

 acl is_website      hdr_beg(host) -i

Where is the “-i” documented. Are there other “switches” besides “-i”?
Is “host” a keyword or fieldname of the request?

Section 7.1 ACL basics:

The following ACL flags are currently supported :

   -i : ignore case during matching of all subsequent patterns.
   -f : load patterns from a file.
   -m : use a specific pattern matching method
   -n : forbid the DNS resolutions
   -M : load the file pointed by -f like a map file.
   -u : force the unique id of the ACL
   -- : force end of flags. Useful when a string looks like one of the flags.