In special times, the authorities issue various rules and laws, which also define how many people can be present at the same time in a shop, restaurant, etc. Violation of these regulations are often punished with a penalty.
One device, iPhone or iPad, is required for each entry/exit to the shop, restaurant etc. The devices are connected via a peer-to-peer WLAN network.
This summarizes the individual registered entrances and exits and enables the control of the total number of current visitors.
There is a warning, on each device, issued when the permitted number of visitors has been reached or exceeded.