The specification at TA = 25°C in the power ratings refers to the total power dissipation of a discrete semiconductor element in an environment with an ambient temperature of 25°C.
In this case, the thermal resistance from the heat source to the ambient air is expressed as Rth(j-a).
The specification at TC = 25°C in the power ratings refers to the total power dissipation when the semiconductor element (case) itself has been forcibly cooled, i.e., when temperature of the package surface is kept at 25°C.
Note that the ratings may include the note "with infinite heat sink". However, in actual use, it is very difficult to make the package surface temperature exactly 25°C, and if you also take derating into account, the allowable power will in fact be somewhere in between TA = 25°C and TC = 25°C.
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