Available from the Functions drop down button only (not available through rules insight)
The List function is a helper that allows quick entry of a range of items for inclusion into a list type control.
"[ItemA] | [ItemB] | [ItemC]"
Once all items have been entered into the editor, and finish is selected, the Rule Builder will automatically place the pipe bar | in between each item and the entire string within quotes " " so it can be displayed in a list type control (List box, Combo box, Option Group).
Once selected the Help and Diagnostics window switches to the editor for entering items for the list.