The Solution frame is the rectangle on the screen representing the size of the final puzzle and that provides a reference for pieces whose position you know. It's where the pieces will get "anchored" if you have the option "Anchor Pieces" activated, and it's also where the “Ghost Image” will be displayed when active.
By default, the solution frame appears at the right top of the board, but you can place it centered on the screen: In the General Options (pressing F3) select “Solution frame centered”.
Additionally you can place the solution frame at any location in the board: Press the spacebar, and when the image of the puzzle is displayed drag it with the mouse to the location where
you want the Solution frame. Then right click the mouse over the image and select “Solution frame here” and there you have it. If you have the "Anchor pieces" activated and have some pieces anchored,
these will move to the new location.