The machine's names (novox/hq ADR 0199, ADR 0223): it holds the node seat node-hostname and owns the two files that say what a machine is called — /etc/hostname and the machine's own lines in /etc/hosts. It was hosts, holding node-hosts-file; the seat was renamed, and the old name resolves to it as an alias.

What it writes

/etc/hostname, whole: the module's hostname setting, and nothing else. There is no default. A machine's name is the operator's: the mesh's name for a machine and the name it calls itself need not be the same, and writing the mesh's name silently would rename a machine. Set it per machine — settings set hostname '{"hostname": "<name>"}' --node <node> — before the module is assigned there; without it the module is left out of that machine's declaration, naming the key. A mesh-wide {"hostname": "${machine:name}"} makes every machine call itself by its mesh name, and a machine's own setting still overrides it.
The machine's own lines in /etc/hosts, as the mesh's marked region at the start of the file: localhost and 127.0.1.1 with the machine's mesh name. Every other line is the operator's, kept byte for byte and given back when the module goes.

When a new name takes effect

At the machine's next boot. The kernel's name is set from /etc/hostname when the machine starts; writing the file changes what hostnamectl reports as the static name and nothing that is running. The module declares nothing that would set it live: a graphical session's X authority is keyed by the name the session started under, so changing it underneath a running session refuses every new window until the person logs in again. Reboot when that is acceptable, or run hostnamectl hostname <name> by hand.

Its verbs

<node>/node-hostname.entries, .add and .remove: the hosts file's lines, each marked whose it is; add one address and its names to the operator's lines; remove a name or an address from them. They change the machine's file and nothing else. /etc/hostname has no verb — it is the setting.