Can a Works With POS app change the POS UI?

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Looking to add a field in POS per line item that would allow us to mark the line item as fulfilled/unfulfilled. Can a Works With POS app "hook into" and modify the POS app UI to add a checkbox next to each product? Or can works with POS apps only exist within their own iframe separate from the checkout UI?

Second, out of the box, if you scan multiples of a product, it shows as one single line with a quantity. Would it be possible to--not always but as needed--break multiples up into two lines so you can mark some quantities fulfilled and some quantities unfulfilled (assuming the first paragraph is possible)?