The spring sensor is contact type sensor.
When a conductive workpiece contacts the sensor rod, the sensor circuit closes and an electric signal flows.
Therefore, this sensor can detect touch of only conductive workpiece.
The cable for spring type contact sensor of US-1 and USD-1 is 6 kinds of color. Blue, Red, White, Yellow, Pink and Orange are prepared. 
(US-2 - US-7 and USD-2 - USD-7 are nothing but Red cable and White cable. )