WOPI means Web Application Open Platform Interface protocol, it can be integrated Office for the web with your application.
For example, if the file host is SharePoint server, you can use OOS open files from SharePoint server, users can edit documents collaboratively with other users using Office.
If you use Office clients to open these files, it does not supports co-authoring.
For more, please refer to "Use the WOPI protocol to integrate with Office for the web", "Integrate with Office for the web" and "Co-authoring using Office for the web".
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