Complex sentence: USDTV channels are broadcast by multiple stations in each area, but through the use of virtual channels they all appear as subchannels of channel 99, even on regular sets that cannot decode them.
Split into simpler sentences: USDTV channels are broadcast by multiple stations in each area, but through the use of virtual channels. They all appear as subchannels of 99.x, even on regular sets that cannot decode them.