By Petter Skodvin-Hvammen - Principal Consultant at Puzzlepart
ContentSource is a managed property that contains the name of the content source used to crawl the current item. The managed property is populated by SharePoint using a hidden crawled property named 315 from the Internal category. You cannot change system provided managed properties in SharePoint Online, but you can change mappings and provide an alias.
Kudos and credits to my colleague Mikael Svenson for his finding and blog post http://techmikael.blogspot.no/2014/09/debug-crawled-properties-and-creating.html
You can import the search configurations files to your SharePoint Online schema at the tentant level from https://contoso-admin.sharepoint.com/_layouts/15/searchadmin/importsearchconfiguration.aspx?level=tenant
- ContentSource-315-Internal_CP_MP_Mapping_Alias.xml: Create the crawled property Internal:315, map to RefinableString00 managed property and define Source as alias for tha managed property.
- ContentSource-315-Internal_CP_Only.xml: Create the crawled property Internal:315
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