The main idea is that these fields capture events that can affect how a claim is paid, along with the time frame for those events. Occurrence span codes identify the type of event (for example, an accident, date of injury, onset of illness, or other qualifying incident), and the corresponding dates show when that event occurred, including the from-to period if it spans a range. Payers use this information to determine liability, coverage rules, and any special processing required for the claim. This helps explain why a claim might be paid differently or require additional documentation. These fields aren’t about patient demographics, diagnosis codes, or discharge status; they specifically provide the timing of events that can influence billing.