I also have the isy994i, but I have not ventured to the Network module. I have the Zwave module...can you have both? Do you know if I can control my Yamaha CXA 5100 via the ISY too? I like to have multiple ways to control everything in case any one way fails.
Sure. The ISY994i has a serial port that connects to the Insteon/X10 PLM modem as well as a slot for the Zwave card. You can operate both from ISY.
The Network Module allows you to send any Ethernet style packet I have ever heard of, using various options for each. It can receive validation responses from packets but not decode them with only the Network Module.
I operate my Phillips Hue bulbs, MiLight RGBWW strings, LEDenet RGBWW controllers, MagicHome UFO RGBWW controllers, MiLight RGBWW bulbs, and Onkyo A/V receiver by dumping character or binary sequences into the module definition strings. I operate many other devices using an RPi with free software to bridge the protocols.
The UDI has had thread discussions how to interface with Yamaha A/V receivers also. I use Onkyo but I understand the protocol is a fairly standard one.
I think the Network module is $49 for the first two years, and $12 per year after that. It come free with the ISY Portal fees (to interact with Alexa and Google Home assistant) but don't quote me on that. It has dropped in price lately.