This tool gives a user the option of either adding their data search results to an existing list or creating a new list. Lists are useful when you are building up a set of data that you want to do something with later on (much like adding things to a shopping cart). Once a list has been created, all the results in that list can be run through one of the processes, opened in ArcMap, exported to a CSV file, etc.
New List Option - Items can be added to a new list:

Existing List Option - Items can be added to an existing list:
