BPMN defines three main types of diagrams: Process, Collaboration, and Choreography, and in BPMN 2.0, Conversation diagrams were also introduced.
Formally, Conversation diagrams don’t present a standalone diagramming technique, but are just a particular usage of Collaboration diagrams.
Conversation diagrams represent a top-level view of BPMN collaboration diagrams. They are useful for representing process landscapes and high-level interactions between involved parties, such as representing an overview of a network of partners and how they communicate with each other.
A participant is representative of a BPMN pool, and represents people, organizations or devices which are involved in a specific communication.
These conversations are related to collaboration diagrams, as a conversation node can be expanded into a series of message flows between participants.
And since participants in a conversation are actually BPMN pools, conversation diagrams can be directly linked to processes via conversation nodes.Business Process Analysis BPMN Video