Who I have the same questioned this topic

Anyone know exactly what this means? I bought an iPhone 8 plus and got an iPhone 8 free. When I go in to pay off the 8 plus this is the message I get. 

*When you bought this device, you got a promotional credit of $699.99. To continue getting this device's promotional credit, your line of service must remain active for 24 months. Promotional credits stop if you cancel service, pay off the device in full or transfer line of service.

You'll lose the remaining $29.16 promotional credit if you decide to pay off this device in full today.