The idea of the Rule Classifier in ENVI is that you decide for yourself what a suitable threshold value should be. So, for example, if you created rule images during a SAM classification, then you might use the Rule Classifier to specify your own rules, based on those rule images, to make a custom classification. You can use the rule images from any of ENVI's classifiers. The threshold values you choose would depend on which type of rule image you are using (in other words, on what the pixel values in the rule images represent). For example, if you output rule images from a SAM classification, and then used those rule images in your own Rule Classifier, then your threshold would, indeed, be based on the spectral angle. That makes sense because your rule images for each selected target in the SAM classification will contain spectral angle values between each pixel and each target. - Peg