This optional feature allows the advertiser to limit the daily number of impressions per source, i.e. a combination of zone, sub1 and sub2. It's available for Advanced campaigns only and affects all sources for all countries. It can be found on the "Budget" step of a campaign:

By default, there is no capping and you can only add whole numbers larger than 1000.

When the campaign has zone daily capping, the first impression on a specific source initiates a counter with a 24-hour lifespan. During campaign selection the value of the counter is checked: if it is greater than the current value of the cap, the campaign is discarded or if it's less than the current value of the cap, the campaign is displayed.

After each selection on that source the counter is increased by 1. Once the lifespan of the counter expires, it is removed and the process starts again

Note: Once created, the counter stays in the system until it expires. So when an existing cap is removed and added back within less than 24 hours, counting will continue from where it was instead of starting from 0.