The Approximate Percentile function is based on an algorithm developed by Greenwald and Khanna. The function gives e-approximate quantile summaries of a set of N elements, where e is the error (the desired accuracy of the approximation). Given any rank r, an e-approximate summary returns a value whose rank r' is in the interval [r - e N , r + e N ]. The algorithm has a worst-case space requirement of O((1/e) * log(e N )).
When running the Approximate Percentile function, you specify e with the Error parameter.