Epson Perfection 640U Reference Manual page 63

If a document contains more than one image, select a target image
by creating a marquee on the image and clicking the Auto Locate
button . The selected target image is located automatically.
target image and other image located and
non-located images
Note:
Create a marquee only on the target image. If the marquee covers other
images, the Auto Locate feature does not work as expected.
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Understanding EPSON TWAIN Features