It’s common to see BPMN diagrams using send tasks to illustrate that we send documents to external entity and receive tasks to model obtaining an answer. Or (which is practically the same) using send message and receive message events for these purposes.
As an example - diagram from Russian Wiki:
Both send/receive tasks and message events are “robots”, automatic actions. If there is a piece of software on the other side to then it’s OK: bytes go into the network connection to be received and processed on the other side.
But buyer is a human, you can’t “call” him as a web service! He communicates with us by phone, by email, by sending paperwork with courier - anyway it’s a communication with a human: our employee would receive email or documents, read them and decide what to do with the information obtained.
- If the process automatically (i.e. pure programmatically, without human intervention) communicate with an external service then use service task, send/receive task and/or send/receive message event.
- If the process automatically sends email then use a script task.
- Otherwise do it with a user task because in the real life buyers, suppliers, partners, agencies communicate not with a process directly but via human participants.