Wifi and Cell service

Wi-Fi is the primary communication and cellular is the backup.  You need the Wi-Fi to get system updates.

You can live with just cellular if you plan on not having wifi and the alarm will function just fine.  Without wifi you lose the ability to run updates like 7 said, but you could routinely take the base station somewhere with wifi to update it.  If you have cameras, you need wifi for those, no way around that.