It's half USB Key, half TRRS breakout... it's the Adafruit TRRS Trinkey specifically designed for Assistive Technology hackers and creators as a simple and low cost, but also flexible and extendable AT device. Many AT interface devices use 3.5mm audio jacks to create switches or variable inputs - often mono TS or stereo TRS plugs. On the TRRS Trinkey, we connect all 6 pins of a 'switched' TRRS jack - tip, ring 1, ring 2, sleeve and the tip switch plus ring 1 switch - to 6 GPIO pins on the microcontroller. That means we can not only detect when plugs are inserted but can change which pins are input, ground or even 3V power. With the use of a stereo/mic splitter, we can have up to 3 simple switches, or two analog potentiometers, or one of each! We particularly like this jack because it has two through-hole contacts near the opening for a good mechanical connection.