A validation rule is a control that checks whether data matches the expected format, type, range, or business logic before a system accepts it. For example, it can confirm that an invoice number has the right pattern, a tax code is valid for a country, or a field is not missing required content.
Validation rules matter because they protect data quality and reduce downstream errors in security, compliance, and operations. In cyber security, weak validation can let malformed input slip into APIs, forms, or integration pipelines, causing corrupted records, logic bypasses, or denial of service. Strong validation helps defenders reject bad data early, enforce trust boundaries, and make automated workflows safer. In systems such as e-invoicing platforms, validation rules are part of the control layer that keeps structured data accurate and usable across different services.