it's written in docs: http://livedocs.adobe.com/flex/3/langref/mx/managers/DragManager.html
It's a string constant means that as a result of drag operation will be a link to source (as a shortcut in windows).
Alternative ways are copy (should create a copy of source object) or move (replace source object to drag destination). Icon near curcor is changed according this status. (see function showFeedback() and styles for different operations)