Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
Really, you can specify any one of them, as any content you purchase is always available to select from within the "cloud" library upon opening up the kindle app on the iphone/pad, and it will then download to the device after that prompt.
If you mean at the time of purchase - you'll install the kindle reader app on each device, and it will take on a name based on the order in which the apps were installed on that device (ie: Nathan's iphone 1, iphone2, iphone 3,etc).
You can rename those by going to the manage content/devices page in your amazon account: http://amzn.to/29Evi9U
From there, choose the entry, then rename. From that point forward, when making a purchase, you can choose the corresponding device where the new item should load automatically. Again though, those purchases are always available on any of your devices at will just by selecting them from the cloud (rather than device) library within the app.