Some apps provide different APK depend on Android version, so it's recommended to select the current Android version of your Android device.
Change this if you have problem downloading some apps (not 100% work). If your device isn't in this list, select "Custom" and enter your device's codename manually, or select a device that has similiar specs to yours.
Please enter the codename showed in the Device ID app. Also you can enter codename of any device. For more device codenames, please view this post. Since this list is incomplete, it's fine if your device isn't in it. You still can use it anyway.