IGBT's Ic (peak current): why always 2 times of Ic

I would like to know:

1) Why, almost on every IGBT datasheet that $I_{cm}$ (maximum peak current) is 2 times $I_c$? - for example see datasheet

How, can I measure this $I_{cm}$, to verify that it is really 2 times $I_c$? Or is there maybe some regulation that on a datasheet that $I_{cm}=2*I_c$ ?

2) Energy losses: I see on the oscilloscope, that there is a tail current after switching. To calculate Eoff, for example how much time do I have to consider the tail current? Some company determine toff for 1us. Why? Some rule specified that time of tail current for Eloss evaluation must be 1us? Or everyone could choose the time that they want?