They can be whatever they want them to be but most of them mirror one of their existing channels.
In NYC there are a few mobile DTV broadcasts. NBC, Telemundo (WNJU), Ion (WPXN) and WMBC have them. There are others but this is what I receive where I am.
NBC broadcasts one of the secondary channels called "New York Nonstop." Ion broadcasts "Qubo" which is their children's programming sub channel. WMBC mirrors their main channel which is mostly ethnic (Korean) programming. WNJU has its main channel (Telemundo 47) on their M/H stream.
Fox had a DTV stream on WWOR which mirrored the main Fox 5 channel. They took it out of the stream. No idea when it will return.
I do not get WLNY. I have no idea what they are showing. When I am in that area I would like to see. But that may not be for a while. I rarely cross 5th ave in Manhattan as it is, and with the price of gasoline I don't think I will be out in LawnGuyLand anytime soon.