Ebook validation is an internationally approved process of checking the code within the books to make sure that they display well on all kinds of devices. The ebook stores only accept "valid" ebooks: your book might look OK on your computer and doesn't work on another device. Ebook validation is here to make sure that all books work on all devices correctly.
Ebook validation rules were established by the International Digital Book Forum. You can use their validator for free.
When you first upload a book, we run ebook validation back-end: we check the book after you've uploaded it, and if something is wrong, we fix the errors before sending the books to the stores. If there is anything we cannot fix, we will notify you and guide you towards the solution.
However, when you re-upload a book, you can immediately see validation results. This is to make sure that nobody accidentally overwrites a right file with an invalid file. As we automatically send your changes to the stores, your re-uploaded ebook file must be perfect. Solutions of most common validation errors can be found here, but we're more than happy to help if you get stuck.