When a SIP method establishes a dialog, it also creates an association within the dialog, known as a dialog usage, between the endpoints. This association contains state that is not shared in the dialog such as expiration, authorization, subscription state, or current session state. SIP was originally designed with the implicit assumption that there was only one usage per dialog. However, a dialog can have multiple usages - for example, when a call is transferred using REFER. Although REFER is a straightforward example of multiple usages, it is recommended that multiple usages be avoided when they can be, since they can create complexity that makes processing difficult. For more information on why multiple usages are tricky and how you can minimize their use, see http://www.ietf.org/internet-drafts/draft-ietf-sipping-dialogusage-06.txt.