The original Stanford talk on posits suggested that they generate an exception and not the equivalent of a NaN. A few months later, I changed my mind and the (unique) NaR bit pattern serves the same purpose as NaN does in floats. We have also learned that the best exponent size (es or eS) is 2 bits, independent of the precision of the posit. So there have been some tweaks, but the basic concept is unchanged.