The current document to copy the selection to another document. And save your selection in the script and also try to use it later. And it must be updated to the new selection. How is that possible work around? I spent a selection information to a text file as a script, I thought the way you need it later loading. (Note that the choice of a simple rectangular shape.)
The selections besides being rectangle with some aspect ratio has some size and position over the document canvas that has it own aspect ratio and size with same resolution as the selection resolution.
Copying from one document to another works best when both documents are the same size that is they have the same aspect ratio have the same resolution settings and same number of pixels.
When documents sizes are not the same Photoshop will do a straight copy of the pixels from the source document to the document being copy to. Photoshop will not do any interpolation. Lets look at what this means.
Lets say you have two portrait 2:3 aspect ratio 4"x6" image where one has a 100dpi resolution and the other has a high 300dpi print resolution. The Portraits are of the same person one smiling the other not smiling. So you select the smile copy it to the clipboard and paste the pixels into a new layer into the none smiling portrait. What do you see. You see a smile that is either three times the size you need or 1/3 the size you need. You need to scale the smile to the scale of the none smiling image.