I parented four cubes, instead of using alt-p to clear parent. I right click parented object in outliner and select unlink. That deletes the parent object and leaves child objects as they are. Right click unlink child object deletes it. So what is unlink command?
To reuse elements of a scene in another scene inside same .blend file or from one .blend file to another, user can create a link between 2 scenes.
As 2.8 is a beta, this was not restricted, yet.
So currently, if you unlink an object used by only one scene, it breaks the link between the object and the scene : as a consequence, it deletes the object.
But if you use Unlink on a collection in a linked copy of current scene, you may experiment that collection is removed in one scene instead of being deleted in both.