Network volume is statistical information that displays data on the volume of traffic and the cost per click (CPC) from various devices in each GEO location for the past 24 hours, taking into account traffic from feeds.
In the network volume, you can see how many clicks were received from your chosen GEO from both desktop and mobile devices, as well as the minimum /recommended/ maximum CPC.
You can use this data for a rough estimate of the expected traffic from your chosen GEO and device, as well as for a rough estimate of the budget needed.