cf_SelectSwap is a Scripts software developed by Dave Konopka. After our trial and test, the software is proved to be official, secure and free. Here is the official description for cf_SelectSwap:

BS Editor: This custom tag produces two list boxes, one with a set of available items and a second for selected items. Users can MOVE items between the two boxes.

The tag wraps up the HTML and JavaScript functionality. It degrades into a regular single select box for users with JavaScript disabled.