App note from Nexperia on an explanation of the derivation of the maximum drain current as a function of pulse duration rating. Link here (PDF)
Historically the IDM capability is shown in the limiting values section of the data sheet for tp ≤ 10 µs. ID at DC (continuous) is also shown. IDM is traditionally specified as four times the theoretical value of ID at DC. The IDM rating here is more than four times the ID as the ID specified is limited by the package, rather than the silicon itself.
It is difficult for the user to know what the capability would be at other time durations. Some information can be gleaned from the Safe Operating Area (SOA) graph, however it is still hard to assess the actual capability for different time intervals.
The aim of this new IDM graph is to provide information regarding the current handling capability of the MOSFET for any time duration in the range provided. It should be noted that only the current carrying capability due to conduction losses is considered. Additional losses incurred due to switching events (linear mode and avalanche) may take the device junction temperature beyond the 175 ˚C limit and cause damage to the device.