In an Itinerary Service you can use a Resolver for dynamically resolving endpoint information and BizTalk Maps. Most of the times when you create a custom Itinerary Service you want to transform or send your message but there are also cases that you want to do something else like logging or validating your message. In that case Resolvers are not useful because the properties and data that they return contain endpoint information and maps. The Itinerary Designer in Visual Studio fortunately allows you to create custom extenders for Itinerary Service model elements that can be used to add properties to the property bag for use by Orchestration-based Itinerary Services. Through this you no longer need to use a Resolver to feed your Orchestration with dynamic data because you can set the properties directly on the Itinerary Service.
For more information see: Creating a Custom Extender for an Orchestration-Based Itinerary Service