In addition to collections and tags, a third way to express relationships between items is by setting up “relations”. Relations can set up between any pair of items in a library (it is not possible to relate items from different libraries).
To create a relation, select an item in the center column and go to the “Related” tab of the right-hand column. Click the “Add” button, and select one or multiple items from the same library in the pop-up window (hold down control or shift (Windows) or command or shift (OS X) to select multiple items) and click “OK”. The selected items will now show up as related items in the “Related” tab, and clicking an item will take you straight to that item.
Note that when you relate item A to B, B will be automatically related to A. But relations are not transitive: relating A to B, and B to C, will not automatically relate A to C.
Some suggestions of how you could use this feature: