The DialogUsageManager(or DUM) is the class that keeps track of all of the Dialog related data structures and sits on top of the transaction layer of the SIP stack. It keeps track of multiple DialogSets which contain Dialogs. Each DialogSet contains all the Dialogs that were created by a common initial request. They all share the same SIP Call-Id and from tag from the orginal request. Inside a specific DialogSet there can be some type of BaseCreator that represents the intial request that generated the dialog. This will only exist on the UAC side. The DialogSet also contains several objects derived from BaseUsage that are using this particular dialog. There are several types of things that are a Usage of a dialog. There can be one InviteSession, one Registration, one Publication, multiple Subscriptions and multiple OutOfDialogRequests. Note the name OutOfDialog is a little weird - OutOfDialog requests are actually in a thing a lot like a dialog but are transactions that are not in one of the other categories. Typically messages that result in OutOfDialogRequests are MESSAGE and OPTIONS.
An initial SIP Request is created by calling the makeX interfaces on the DUM. It is then sent using the send interface on the DUM. When a response is returned for the request, a callback from one of the Handler classes will be called to notify the application about the incoming events. This will pass up some type of client or server usages class that can be used to send aditional messages and responses in the context of this particularr usage of the Dialog.