CMS HCPCS modifier codes:
https://www.cms.gov/Medicare/Coding/HCPCSReleaseCodeSets/Alpha-Numeric-HCPCS.html
AMA CPT modifier codes:
https://www.ama-assn.org/practice-management/cpt/finding-coding-resources
Ambulance modifier codes:
https://www.cms.gov/files/document/origin-and-destination-codes-specific-ambulance-service-claims-and-emergency-triage-treat-and.pdf
Null/missing = Source value is missing or unknown