From what I understand, the HD01/HD02 feeds, which are complete, are used for feeding any programming "live" that is not a special event. Shows can be recorded off of this feed to air at a later date, and I assume they are in many cases, but it is padded with promos and whatnot anyway. There is no feed of the News Hour at 7PM Eastern, for example, other than this feed.
HD03 shows the News Hour an hour early, Charlie Rose an hour early, etc. I believe this to also be where live News Hour events are aired, but I have yet to actually watch one here. This one goes to bars and tone.
HD04 contains all the oddball shows that aren't provided directly by PBS, so the ones that are produced by the member stations and distributed across the country. Again, this one goes to bars and tone.
- Trip