The Paste Into command acts in a similar method to the Paste command. The primary difference becomes apparent if there is a selection within the canvas. Unlike a Paste which will merely center the pasted image data over the selection and remove the old selection replacing it with its own, Paste Into will clip the pasted data using the existing selection. The new selection can be moved as per normal, but will always remain clipped by the original selection area.
If no selection exists, the command will place the data from the Clipboard in the center of the canvas as per the Paste command.