From the SKM Manual:
Reduce Generator / Synchronous Motor Fault Contribution To Generators and synchronous motors do not supply the same amount of fault current after a certain number of cycles following the fault. For example, the fault current may be reduced from the initial 1000% of the Rated Current (10 per unit) to 300% after 10 cycles. Enter the percentage of the Rated Current and the number of cycles after which to reduce the fault current to. PTW assumes a step change from the initial fault current to the reduced value and incident energy will be calculated using the initial fault current and the number of cycles specified, then accumulated with rest of the incident energy calculated using the reduced fault current and the duration at which the protective device trips. The Apply To Generator check box controls whether the reduction of contribution should be applied to generators. If unchecked, generator contribution will be the same as the initial fault for the entire arcing duration. Similarly, the Apply To Synchronous Motors check box controls whether synchronous motor contribution should be reduced after the number of cycles.
