You may drag and drop items at any time. You can drag items within an outline, or drag items between outlines.
To drag an item click and drag on the triangle handle that's to the left of the item. When you are ready to drop you have three options:
Move is the default drop behavior. The item (and child items) move to the drop location.
Copy drop behavior creates a copy of the dragged item (and child items) at the drop location. On OS X to do a "copy" drop hold down the Option key while dropping.
Link drop behavior creates a hyperlink from the drop location back to the original dragged item. On OS X to do a "link" drop hold down the Control key while dropping.