Every time you create a contractual relationship with a trading partner, you must be able to meet the requirements of that contract, including payment terms, discounts, delivery modes and requirements, and so on. All these factors constitute your relationship with your trading partner. Each relationship will be different.
Your BizTalk server should have the ability to track all aspects of this relationship as they pertain to generating and processing transactions between each partner and your company.
A BizTalk server should also be able to differentiate between business partners that you are currently trading with and those that you are considering trading with. For example, if you are negotiating with a potential partner, you might want to set up a pro forma relationship in your BizTalk server to test whether the parameters mandated by the trading partner agreement will fit into your system's workflow-management module.