Files that contain the .rels file extension are relationship files. These files contain information about how the parts of different Microsoft Office documents fit together. This information is also referred to as relationship parts.
RELS files serve as markers for the documents that they pertain to. They allow the various Microsoft Office applications to utilize the information in one file so that it can be used in another file.
The RELS file format is only compatible with the more recent versions of the Microsoft Office applications which support Open Office XML documents. The RELS files are saved in the directory of the Open Office XML document.
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If you have additional information about the RELS file format or software that uses files with the RELS suffix, please do get in touch - we would love hearing from you.