That's the port that Square uses for transmitting data. ;-)

https://stackoverflow.com/questions/2181476/bandwidth-from-h...

Wait until you learn that some phones have serial access through the headphone jack.[1][2] I don't know much about it yet, but I assume it can be exploited in some way, under some circumstances.

[1] DOWN THE RABBIT HOLE AND BACK OUT AGAIN: SERIAL OVER HEADPHONE JACK https://hackaday.com/2016/10/21/down-the-rabbit-hole-and-bac...

[2] A better audio jack console cable for Google Nexus devices http://www.pabr.org/consolejack/consolejack.en.html

Ignorance is bliss