Events of this type are used in case of receiving messages from participants or sending messages to participants. Triggered by transmission or receipt of messages, these events affect the process. If they are waiting messages, they let the process continue. Upon exception handling, they play roles to switch from normal flows to exceptional flows.
In case of Catch Event, i.e. when it indicates "receipt of messages," a marker shown in the top figure on the right is used.
In case of Throw Event, i.e. when it indicates "transmission of messages," a marker shown in the bottom figure on the right is used.