If you have an iPad or iPhone, and want to know how your eBook is going to look on those devices when someone buys the book and downloads it, you need an AZK file. You can find out more about AZK files, how to create one and how to view it, in our article here in help, Do NOT put your MOBI file on Your iPad To Review!

But, Do I Upload That File, at the KDP? How Do People With iPads Get the Right File?

Fear not—you don't upload the AZK, or an AZW, or an AZW3. Those are all file types that really have nothing to do with you as a publisher. At Amazon, you'll upload a single file, typically a MOBI, sometimes, a KPF file.  Amazon processes the file and then correctly delivers the right file format to the correct device, when a buyer asks for the eBook to be delivered to his or her device. This is why Amazon requests the device name, when you ask for a file to be delivered from your Content and Devices.  Again, this is Amazon's problem, not yours.

The AZK file exists so that you can accurately preview how your eBook file will look on an iOS device. It exists solely for the purpose of that preview, and not for any other reason. It's not uploaded, or used, for any production purpose other than your previewing same, as the publisher.