Touch sensor is best used for user input, but can also be used as a limit switch. 
Configuration
The REV Touch Sensor is a digital sensor that is wired to the N+1 channel on our standard 4-pin JST PH connector. In other words, the REV Expansion Hub digital ports are grouped with two input/output channels per port: 0-1, 2-3, 4-5, 6-7. In general we refer to each channel as N and N+1. When pressed, the touch sensor pulls the digital channel low from its default high state.