Normally, a knowledge article offers helpful instructions for all people who rely on the service for which the article was written, regardless of the instance that people use of this service. That is because each instance of a service typically provides the same benefits or capabilities.
There are some cases, however, where one instance might be slightly different from the next. Perhaps because of small regional variations, specific options that have not yet been rolled out to all service instances, differences in the infrastructure that an application service is running on, etc. In such cases, it can be useful to limit access to a knowledge article to only those people who rely on specific instances of the service that the article is linked to.
To make it possible to limit the visibility of a knowledge article to only the people who have SLA coverage for specific instances of a service, it is now possible to link service instances to a knowledge article.
If a knowledge article is not linked to any service instances, it is considered useful for anyone who relies on any of the instances of the service for which the article was written. As soon as one or more service instances are linked to an article, the article can be found only by the people who are covered by an active SLA for one of those service instances, plus the specialists who support these people.