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sub-domains isolate the link value and don't allow for the flow of value into the main domain. It's best practice to use sub-folders instead on the main domain that allow for link value or 'juice' to spread through the site - thus - building trust. Unless a sub-domain could hold and contain it's own content to work apart from the primary root and gain trust and value, I would go the route above. Every situation needs to be clearly identified and reviewed.