The Link domains setting is applied to a ConversionRuler Site setting and can be used to improve site visitor tracking and consistency.
By default, the User ID is stored in the browser's cookies, which means it can only be accessed by pages on the same domain or subdomain. To track the same User across domains, you can specify the domains which you would like to track in the Link domains field.
A valid value for this would be something like:
Which would modify links on all web domains which end with the values:
Any links on your site which connect to pages on these domains will be automatically updated to pass the User ID to the target domain.