In the “Relations” tab you can see and/or add relations between entities. You can see a description of the different types of relations here.
How to add Relations?
At the top of the “Relations” tab you see an input field. If you want to create a new relation from the current entity, start typing the id or title of the entity, to which you want to create a relation.
A selection list with items matching your input so far will appear below the input field. The matching entities are grouped in the different relation types. Select the entity, you want to create the relation for.