IIUC, the "homie/" prefix can be changed to "X/" for branding reasons
If this root topic does not suit your needs (in case of, e.g., a public broker or because of branding), you can choose another.
However, the online validator will fail if this is changed:
reactor/device123/$homie 4.0
reactor/device123/$name My device
reactor/device123/$state ready
reactor/device123/$extensions
reactor/device123/$nodes mythermostat
reactor/device123/mythermostat/$name My thermostat
reactor/device123/mythermostat/$properties temperature
reactor/device123/mythermostat/temperature 22
reactor/device123/mythermostat/temperature/$name Temperature
reactor/device123/mythermostat/temperature/$unit °C
reactor/device123/mythermostat/temperature/$datatype integer
IIUC, the "homie/" prefix can be changed to "X/" for branding reasons
However, the online validator will fail if this is changed:
Fails with