3.5mm Jack

A place to plug in a standard mono audio plug on an AT Switch.  These look like Aux cables (or headphone cables for a phone), but are actually Mono, not Stereo and have only two contacts.  One on the Tip and one on the Sleeve.  They are sometimes called “TS” jacks or plugs for “Tip and Sleeve” as compared to Stereo versions which are called TRS “Tip Ring Sleeve”.