Following, there’s an excerpt from Roon’s knowledge base about zone grouping to probably answer your question more elegantly than I could put it into words…
Roon supports Zone Grouping , or synchronizing audio playback in several zones within your house. Zones can only be grouped with other zones of the same type , meaning Squeezebox zones cannot be grouped with RAAT zones, Meridian zones can’t be grouped with Airplay zones, etc. Not all technologies support grouping, but the most commonly used options do. These are the types of zones within which grouping is possible: