Frequently the current rating, Io, of a rectifier assembly contains the following info:
A typical rating might include -
Io, 70°C in oil
Io, 55°C heat sink
Io, 25°C no heat sink
The current rating in oil assumes 70°C is the temperature of the oil, and that the oil is circulating.
A temperature of 55°C assumes that is the temperature measured at the bottom of the heat sink, opposite the site of attachment to the assembly.
An ambient temperature of 25°C assumes that is the temperature of the air, not the case temperature.