T-7910 CPU FAN HEADER - WATER COOLING

I am trying to understand the 7910 CPU FAN HEADER PINS. I have attached three pictures.

7910 CPU FAN HEADER   ( 5 PINS ) ( 1. Tachometer 2. +12V 3. GND 4. PWM 5. KEY ? )

STOCK COOLER CONNECTOR ( 4 PINS ) ( 1. Tachometer 2. +12V 3. GND 4. PWM 5. NOT USED )

WATER COOLER CONNECTOR ( 5 PINS ) ( 1. Tachometer 2. +12V 3. GND 4. PWM 5.  ? )

SO WHAT IS PIN 5 ?