Clustering is the most frequently used automatic thresholding method. Use the clustering method when you need to threshold the image into more than two classes.
Clustering sorts the histogram of the image within a discrete number of classes corresponding to the number of phases perceived in an image. The gray values are determined, and a barycenter is determined for each class. This process repeats until it obtains a value that represents the center of mass for each phase or class.
Also, check out the in-depth discussion section here.
