The devices in question as their original counterparts might have the same features like processor, iOS version, storage memory, etc. But comes with different configurations, firmware, and package (mostly boldly written 'Not For Sale').
The altered firmware is often the most visible feature these types of iPhones have. Most of these devices display a commercial video playing continuously showing things the device has to offer. Demo devices through the action of its firmware are heavily restricted devices, limiting access to very common features present on its ordinary type.