[avahi] What triggers a service to send it's information

[avahi at radolfstrasse.de]

Hi everyone,
I am trying to understand the mechanics of avahi and wonder what makes a
service to send it's information.

I am currently looking at the network traffic using wireshark/tcpdump.
After starting avahi-daemon and avahi-discover  I see in the network log
that my photo-voltaic inverter sends a "Standard query resonse" containing
its name "_http._tcp.local"  and domain name "pvserver._http._tcp.local".

But I cannot understand what triggers that embedded system to send it's
information.

Thanks for any insights.
Tobias