Moneropedia
Locally-unique host
The Basics
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.
In-depth information
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.
Notes
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Monero primarily uses canonically unique host resolution while I2P only uses locally unique host resolution.
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I2P's and Kovri's assigned top-level domain is currently
.i2pand Kovri intends to only process/use the.i2ptop-level domain