A Column or other non-aggregate expression can be moved over the (empty-Value Expression) branch of the tree. A numeric literal value can be moved over the (empty-Percentile) branch of the tree, representing a value from 1 to 100, indicating the percentile to use.
In addition to specifying these values, the user can also specify whether distinct values only should be used (discarding values that are multiples), and an interpolation scheme to use when deciding between multiple solutions. These options are enabled through the Properties panel as shown below. Double-click on Percentile, or highlight it and click Properties.