This has to do with something called "Stream Management". Live TV, Pay-Per-View, U-verse Movies, On Demand, Recording TV shows, and applications come as streams of content to the TV receivers. We provision each household with a maximum amount of network bandwidth based on the capacity of the network and the distance from the home to the serving terminal in your neighborhood. Stream management limits the number of streams so that it won't exceed the available bandwidth. Each U-verse TV customer has a total of 4 streams, they're broken down into HD (High Def) and SD (Standard Def) streams.