RELS, Open Office XML Relationships File (.rels)
The RELS Page Layout file format
File with the RELS extension can be found within Open Office XML documents. They contain information about the relationships between different parts of the XML document it is contained in. RELS files are saved in their own directory within the Open Office XML document: "_rels".
Technical details of RELS files
RELS files regulate the relationship of parts contained in an XML document. More precisely, the RELS files contain specification that tell the XML document in which way the different parts are connected to form the resulting document. This information is stored using the XML format and appropriate syntax.