Purpose of the "Linking" attribute of the topicref element

Hi SDL-KC Experts,

What's the purpose of the "Linking" attribute of the topicref element?

The doc lists the allowed values, but doesn't explain the effect of each value.

LinkingSpecify the value of the @linking attribute. You can select from the following values: (None), Normal, Source only, and Target only.

Regards,

-Kumar