Creepage distance get from UL840 note
The difference between 6.1 and 6.2 is based on the continuous voltage differential between the traces.  

For example: If you expect a continuous 1000V between two traces, it should be spaced according to 6.1.

If you only need to be able to withstand a 1000V surge infrequently and/or for brief periods of time, then the spacing can be according to 6.2.