RNC is an extension specific to files created with the RELAX NG schema language for XML. RNC files contain program code written in the RELAX NG language. Very often, these files contain syntax like in XML, but can also be written in the form of so-called. Compact syntax.

Technically, RNCs are plain text files and can be opened and modified with any text editor. The RELAX NG language was developed by the OASIS consortium.