How many terms are possible? There are four basic ingredients: TP, FP, TN, and FN. So if each term may or may not be included in a sum in the numerator and denominator, that’s 16 possible numerators and 16 denominators, for a total of 256 possible terms to remember. Some of these are redundant, such as one(a.k.a. ONE), given by TP/TP, FP/FP, etc. If we insist that the numerator and denominator be different, that eliminates 16 possibilities, and we’re down to a more manageable 240 definitions. And if we rule out terms that are the reciprocals of other terms, we’re down to only 120 definitions to memorize.
And of those, John points out the handful which are generally important, providing us an excellent table with definitions of commonly-used terms.