KDP takes the first 10% of your content and creates a sample that customers can view before purchasing the Kindle Edition of your title.
The 10% is 'rounded up' - this means that if your content consists of five images, for instance, the sample would consist of one image (it wouldn't cut off half the image).
Similarly, KDP doesn't cut a paragraph or a sentence in half. The logical structure of an element of your content is allowed to finish (such as going to the end of the paragraph).
Content that was uploaded to KDP and had Kindle Editions created before the introduction of sample functionality has been modified to include samples. This means that all of your old KDP books now have samples on their product detail pages!
If you have any further questions about KDP sampling functionality, please write to our support team by clicking the "Contact Us" button.