There will be 3 sensors generally on motherboard, One is under the processor take it as core temp.
Second one will be next to processor socket at the place of RAMs and third one will be at the place of chipset.(Generally this one will be auxiliary temperature)
These second and third places may be different according to the motherboard. Check your motherboard sensor position on the booklet given with it.
AUX temperature is typically the Ambient Case Temperature. It's a small diode located on the motherboard. Modern day CPUs and GPUs would be reported as CPU and GPU due to on-die sensors.