QuantileOperator produces the value representing the data at a cummulative probability. This is a numeric operation.
As an example, quantiles might be produced for the 0.25, 0.5, and 0.75 probabilities (alternately, the 25th, 50th, and 75th percentile ranks, the 50th percentile being the median). Data is sorted is ascending order. This operator takes one percentile, but it is common to generate multiple quantile ranks for the same field when summarizing.
All the field's values are stored in memory as part of this calculation. This is handled by unique key value lists.