I ran into the same problem several months ago when we were looking into staying at the Swan. We had a previous reservation for the Poly, made through DreamsUnlimited. Because of some very unexpected circumstances, it was necessary for us to consider other resorts.
When I asked my Dreams agent to check availability at the Swan or Dolphin, she told me that she could not do that, that I would have to go through the DreamsRes system (which is part of DreamsUnlimited, but doesn't include all the trip planning help). I was very confused too and ended up making the reservation myself (with the extremely kind and thoughtful assistance from a fellow DISer!)
Since then it has been explained to me this way: the DreamsUnlimited agents assist you in all levels of planning a trip, including itinerary help, PS help, daily suggestions, etc. The DreamsRes system (and I never understood completely if there were specific DreamsRes agents) is part of
Dreams Unlimited and (I think) may offer assistance with PS's but that's it. You make the reservation through their website.
If anyone has additional information or a clearer explanation, I would love to hear it! Since all this happened back in January, I've seen numerous posts about people using Dreams to book the Swan or Dolphin and I've always wondered if I was given wrong information or if they're really talking about using DreamsRes instead of DreamsUnlimited.
Kelly