A Conversation is a simplified version of a Collaboration, and has access to the same modeling elements. A Conversation defines a set of logically related message exchanges, where the message exchanges are related to each another reflecting a distinct business scenario, e.g. a request followed by a response.
A Conversation has two other graphical elements not available in any other BPMN diagrams:
•Conversation node elements (Conversation, Sub-Conversation and Call-Conversation)
Participant / Pool