As far as I know, if you make a link type available on a work item, both endpoints are displayed in the Links section of that work item.
So, for example, if you make the parent-child link available on a Change Request, the Link Types drop-down menu will let you select both "parent" and "child" options.
Is it possible to limit the choice to only one endpoint?
For example: it would be nice to make only "parent" available on the Change Request and only "child" available on the Task.