As a little bit of background to HA Discovery from HE . Basically there is a MQTT topic, usually 'homeassistant' that HA watches where devices advertise they are available via MQTT . You configure this topic in both HA and my app.
The 'advertisment' is simply a single config message that tells HA what the device is (device type - within the topic) and what the MQTT topics are that provide for status and control of that device. It also tells HA how to translate (template) the payload values from HE into values that HA will understand e.g. a 0-100 range to 0-255 and how to do the reverse so that HE can understand HA's commands.
In my usage I am sending this information to MQTT using the homie protocol but this isn't of any relevance to HE so the advertisement messages just serves to identify specific topics that HA can get the appropriate information from.
So the 'advertisements' are just pointers to the topics and information on how to 'template' it usefully.
