After the Buyer has decided what they would like to buy, they need to initiate a Propose Corda Transaction. The Corda transactions are shown as a set of duplicated actions in the swim lane of each participant involved in the Corda transaction, with the Corda logo in the corner.
The actions are joined with a two way dashed line. In BPMN dashed lines denote messages and are normally unidirectional. However, in the CorDapp BPMN a two way arrow is used to reflect that there is a series of back and forth messages which result in a finalised transaction on the ledger.
The party that initiated the transaction is marked, and the message line is annotated on the Initiator end with a blue box giving more details about how the transaction was formed. This is typically what Flow is invoked and what the Command in the transaction should be.
In this example you can see that the Buyer will initiate the ProposeFlow to create a Propose transaction.