Minecraft wiki entry for clone, associated text: Specifies the lower northwest corner of the destination region.
In Java Edition, it must be a block position composed of <x>, <y> and <z>, each of which must be an integer or a tilde and caret notation. In Bedrock Edition, it must be a three-dimensional coordinates composed of <x>, <y> and <z>, each of which must be a floating-point number or tilde and caret notation.