Understanding the Incident Identifier
An Incident Identifier is typically a unique number, perhaps a counter for the Nth incident of the year. (Incident numbers are often reset at the start of the year.) The Incident Identifier is often generated at the dispatch center (perhaps via the local CAD - Computer Aided Dispatch system.)
NOTE: If you don't have an incident identifier or CAD number readily available, leave this field blank.
NFIRS Incident Number
For NFIRS the incident identifier is a 7-digit number that is unique to the day it is toned, but since few departments run ten millions (9,999,999) calls a year, a yearly unique identifier is easier to work with.
If in doubt, leave blank ...
If you don't know the incident identifier, leave it blank. Sometimes an administrator can communicate with dispatch after the incident to get the incident number.