1. A method for defect classification comprising: storing a definition of a region in a feature space, wherein the definition is associated with a defect class and comprises a kernel function, wherein the kernel function comprises a kernel parameter that is associated with a plurality of parameter values, wherein each of the parameter values for the kernel parameter is used to determine a different shape of the region based on the kernel function and corresponds to a different desired rejection rate for automatic classification of a plurality of unclassified defects for the defect class; receiving, by a processing device, a confidence threshold that corresponds to a desired rejection rate for automatic classification of at least one defect associated with the defect class; selecting, by the processing device, one of the parameter values of the plurality of parameter values for the kernel parameter that determines the shape of the region and corresponds to the desired rejection rate associated with the confidence threshold for automatic classification of the plurality of unclassified defects; receiving, by the processing device, inspection data for the plurality of unclassified defects detected in one or more wafer samples under inspection; automatically classifying, by the processing device, the plurality of unclassified defects for the defect class using the kernel function and the selected one of the plurality of parameter values of the kernel parameter that determines the shape of the region and corresponds to the desired rejection rate for the plurality of unclassified defects to generate a first plurality of classification results, wherein the plurality of unclassified defects are classified with a class label associated with the region based on a location of the unclassified defects in the region with the shape determined from the kernel parameter with the one of the plurality of parameter values that is selected based on the confidence threshold that corresponds to the desired rejection rate; obtaining a second plurality of classification results for one or more of the plurality of unclassified defects for the defect class; and combining the first plurality of classification results and the second plurality of classification results.