The file extension RELS denotes a Microsoft Office Open XML Relationships File. This relationship file contains XML-specified information about how parts within the XML document interact with each other. Rels files are usually stored in subfolder '_rels'.
Since RELS files are written in XML, these files can be opened and edited in any text editor. For a structured view you should better use an XML editor. Because RELS files only contain references to other files, they are not usable individually.
RELS files are part of the new Open XML standard that Microsoft introduced in 2007. Rels files always belong to a certain office document, eg. Word docx, Excel xlsx or PowerPoint pptx and are usually embedded in this. In order to be able to view or edit the RELS file of an Office document, it must first be extracted from the document. If you open an arbitrary Office document with a Pack program (or change the extension of the document to zip) you will find the RELS file in the subfolder '_rels'.
Most often resolving problems with opening RELS files is very simple. Just install an appropriate program that supports such files. All of the listed programs support RELS files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of RELS files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate RELS files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Microsoft Word applications is not on the list, select "Browse" option in order to manually locate the directory where Microsoft Word has been installed.
Sometimes problems with opening RELS files may not lie with the application itself, but can arise due to other causes, such as: