If the Ipad is (wireless) connected to an AIS-receiver, the names of other vessels, which are equiped with an AIS-transponder, can be seen on the chart.
Those vessels will be presented in a list as well.

To realize this the AIS-receiver has to be connected to a serial WIFI-convertor.
A cheap and excellent device is the USR-WIFI232-602..
This device works without any problems with App4sail.