A rejection fraction identifies the portion of cells that will remain unclassified due to the lowest probability of correct assignment. The default is 0.0, meaning every cell will be classified. There are 14 predefined values for the rejection fraction: 0.0, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 0.75, 0.9, 0.95, 0.975, 0.99, and 0.995. If a specified rejection fraction is between two valid values, it is assigned to the next upper valid value.
Make sure that the rejection factor is 0.0 or greater, and less than 1. Check the Maximum Likelihood Classification documentation for further details on this parameter.