Tax agencies and government offices send document-request letters (e.g. "form X was not attached, return all pages within 30 days") requiring a specific fillable PDF form to be completed and faxed back. Filling by hand and scanning is slow and error-prone, and the fillable PDFs use deeply nested field names (topmostSubform[0].Page1[0]...) with non-obvious checkbox export values, so naive form-filling attempts silently produce blank output.