Here's the (to be updated) list of Mac temperature sensors supported by the app.
Airport Card - temperature of the sensor on integrated Wi-fi module
Ambient, Ambient 2 - surrounding temperatures inside mac
CPU Core 'X' - momentary temperature of one of CPU (Central Processor Unit) cores. Sometimes it's not the best source for sensor-based control because of intense fluctuations.
CPU Diode - temperature read from CPU die sensor
CPU Proximity - temperature read from a sensor located right next to CPU
DIMM Module 'X' - temperature read from a RAM (operating memory) slot
Optical Drive - temperature read from Optical Drive sensor on iMacs with optical drive (models prior to Slim Unibody iMac 2012). This sensor is responsible for cooling down Graphic Card, not the actual optical drive
GPU Diode - temperature read from graphics card die
GPU Heatsink - temperature read from graphics card heatsink
HDD Proximity - temperature sensor near hard drive on some iMac models
PECI GPU - temperature from Intel integrated graphics chip
Drive Bay 'X' - temperature read from a 3.5 drive bar number 'X' on Mac Pro (2006-2012)