With respect to their influence, USU can be either correlated or uncorrelated, depending on the circumstances. Fully correlated USU will determine the lowest possible accuracy to which it is possible to determine a certain physical quantity by a particular experimental technique due to unknown bias-inducing effects. Uncorrelated USU may contribute in a quasi-random way to unexplained scatter in measured **data** when it is assumed by the experimenter that the measurement procedures are well understood and under control. This may occur because of unrecognized instabilities in the experimental apparatus. Uncorrelated USU will certainly limit achievable precision, and possibly the attainable accuracy as well.