Which temperature element is causing MacBook to stop? I have a MacBook 12 Early 2016.

When I'm using it outside and it is hot, it warns me about stopping because the temperature is too high. And then it stops.

I'm using a tool to see the temperature of different elements of the MacBook but I would like to know what is the element that makes this warning : I don't think it is the CPU because sometimes it is hotter without any warning. 

Is it the battery temperature or cpu proximity ? is there an official explanation ?

Thanks
No. Apple doesn’t publish firmware code and you’d have to reverse engineer several components including the on die cpu sensors, the discrete sensors, probably sensors on storage and possibly dozens of sensors within the battery cells. 

Since the MacBook have no blowers, on a practical level, you can prop it up on a stand and ensure airflow is good and optionally ensure cool air surrounds the device to reduce internal temperatures. The whole case is a heat sink and radiator, so you can dramatically cool it with an elevated stand and/or cool airflow. 

If you have any coverings or cases on your Mac - absolutely remove them since a thin gap of air between the metal and plastic is a great insulator just like a blanket would be.

Outside in the sun or even with shade, you can have several additional problems:


sunlight on the black screen will overheat it dramatically and light on the rest of the case is less than ideal. 
getting cool air will be hard or impossible