A locally-unique host is a FQDN defined by you and resolved only by you; similar to how a hosts file is implemented. Not to be confused with Canonically unique host.
You have the option to share your interpretation of how the host is resolved (e.g.,
localhost always resolves to
127.0.0.1) but the resolution is not canonically enforced (e.g., someone else can map
localhost to any arbitrary IP address).
Hosts in a public subscription can be considered Canonically unique host's within the I2P network but, ultimately, you are free to re-define them as you wish.