Relax NG (Relax Next Generation) is an XML schema language. An RNG file contains a schema written in the Relax NG compact syntax. This syntax is designed to be more human-readable and easier to write than the XML syntax of Relax NG. The schema defines the structure and content of XML documents. It specifies the elements, attributes, and data types that are allowed in a valid XML document. RNG files are used to validate XML documents, ensuring that they conform to a specific structure and data constraints. They are commonly used in software development, data exchange, and document processing to ensure data integrity and consistency. The compact syntax uses a more concise and readable notation compared to the XML syntax, making it easier for developers to create and maintain schemas. RNG schemas can be used to validate XML documents programmatically using various XML processing libraries and tools. They are a powerful tool for defining and enforcing data standards in XML-based applications.