For a long time I never worked very much with Parties and Role Links in BizTalk and I've been teaching it some lately (and had a question posted on a board) so here goes. I really just plan to talk about them and tell you where to get more info on using them. I guess it started when I was just trying to touch all the features I never had and found Role Links in the Orchestration designer. I checked MSDN which does give the answers, but not as directly as I was looking for (in retrospect the documentation is great, but I just needed a good overview to understand what I was looking at).
As I looked more I came to Richard Seroter's blog posting here which also points to a good post by Todd Uhl. This all started to make more and more sense over time. Then I had the requirement for a solution to send messages to a large variety of partners depending on some information that is looked up based on the input of an incoming message. There are a lot of ways to solve this in BizTalk and Dynamic Ports is certainly one, but I really don't like dynamic ports because they obscure important solution details from the administrator / operations team. To make matters worse the number of possible partners was likely to be in the hundreds for this solution.
After yet more research and playing around I stumbled onto the Party Resolution Sample (it's in the older versions too despite this link now pointing to 2009) and this is where I had the epiphany. Parties and Role Links could solve my problem with almost no work on my part. Here was my chance to look like a superhero without needing to do much more than figure out how to tie the pieces together (something that I find is common in BizTalk).
Basically it works like this: you use Role Links in your Orchestration like Richard suggests and if you have the defining piece of information in your inbound message (like in Richard's case) you're set. All I did was have a simple external service lookup that provided me that data from an existing repository. The best part of this solution is that the Maps go on the Ports (like they always should) and the Orchestration simply uses the internal (canonical) schemas. The other than assigning/resolving the party in the expression shape the Orchestration knows nothing about the ports and their details.
There are a few caveats such as: the Parties must exist in the BizTalk Group (surprise) and that they must have ports tied to them which will become a requirement of application (or more precisely Orchestration) binding before everything can be turned on. Also I'm pretty sure the ports need the same basic communication pattern (One Way or Two Way).