The free target is a special UI element to add to the editor to have custom user interface. Usually this is a column to the right or a row at the bottom of the editor.
The data element here would be either:
a size object with width and height - see Size for details.
a number defining the width or height (if it is a column or a row) of the free target.
The editor Ui data flag identifies a UI element within the editor to change data for.