in TreeNode, what is the difference between add and move traits

I've been looking at the source and I can't figure out what the difference
is between the 'add' and 'move' traits in a TreeNode. I am aware the the
'move' trait can be a list of tuples to convert type, but if you just gave 
it a singleton, it seems identical in all respects to 'add'.

Can someone please enlighten me?

thanks,
D