There’s been a major incident. A tactical commander is despatched to the incident zone and ordered to report on the situation. She sends out field agents with instructions to gather information according to pre-defined criteria known as CHALET. She wants to know about casualties (the number of dead and injured), hazards (such as unexploded bombs or high radiation levels), access (the best routes in for emergency vehicles and routes out for evacuations), location (where exactly has the incident happened?), emergency (are fire services needed as well as ambulances?) and type (chemical spill, earthquake, explosion?). As the information streams back, she begins to assemble the story of what’s happened from pre-defined inputs. See how London’s emergency services work together –
http://www.leslp.gov.uk/docs/major_incident_procedure_manual_8th_ed.pdf