Definition: The Maximum Power Rating is defined as the maximum power that a PN junction or diode can dissipate without damaging the device itself. Usually, it is specified by the manufacturer in, its datasheet. The power dissipated at the junction is equal to the product of junction current and the voltage across the junction.
If the power developed across the junction is more than the maximum power dissipated by it, the junction will be overheated and may be destroyed.
Mainly the data sheets provided by the manufacturer, the maximum current rating that a diode can handle is only given. As it is more convenient to measure the maximum current and to design the device accordingly