"Tctl is the processor temperature control value, used by the platform to
control cooling systems. Tctl is a non-physical temperature on an
arbitrary scale measured in degrees. It does _not_ represent an actual
physical temperature like die or case temperature. Instead, it specifies
the processor temperature relative to the point at which the system must
supply the maximum cooling for the processor's specified maximum case
temperature and maximum thermal power dissipation." --amd documentation via k10temp documentation
It's not fresh on my mind, but you can essentially think of it as a ratio. If you're seeing 25c it's not really a temperature. It's more like 25/tjmax*tdp is needed to be removed by the cooling system.
Or another way to think of it, if you have a cooling system capable of 100Watt on a 100W tdp cpu then 25/tjmax can be used for setting the fan speed ie: 10% 30% 50% etc.
I know this isn't entirely right, the formulas/algorithms are more complex.
here's an article that seems a little more detailed
https://yeupou.wordpress.com/2012/02/18/getting-accurate-temperature-reading-for-the-cpu/
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