It indicates that the process is triggered when a signal is detected. This signal was broadcast communication from a business participant or another process.
There are two types of Signal Intermediate events: throwing and catching. This is a consequence from a trigger which is a signal what can broadcast or receive.
Sign Event can operate across Proccess levels. For example, a Signal Event could trigger status reports to a customer, indicating that the process had reached and agreed milestone.
In this case, a signal just arrived before it task "display flight reservations" will be executed.
Objects: Business Process Diagram
None Event
Time Event
Conditional Event