By default, the ISBN will be attributed by us for free and will be printed on the back cover.
If you have your own available ISBN (not used by any other book), you can of course use it. Just choose the right option and enter your ISBN when filling the POS request:
You can't however use the eBook's ISBN as the same book in its digital and printed versions require two separated ISBNs, as they are two different products. You can read more about ISBNs here.
If you previously published a paper version of your book with an Publishing house, you can't use their ISBN for POS, as they own this ISBN and it's legally and technically linked to this specific publisher.
in the event that, after you activated your book on POS, you want to print copies of your book with the printer of your choice, you can use the ISBN we assigned to your book if, and only if, the format and content of the book you print with the printer is exactly identical.