For every table and view in the database, it is possible to open a properties dialog which contains a subpart (the lower part of the dialog) to manage relations between table's fields.
When a table (or view) has its properties box displayed, all the information is centered on that table (or view): the presented relations are the ones which are about a field of the table in them. The relations edited in one properties box, if they also appear in another properties box, will be automatically updated, so all the information in all the properties are always up to date.
To manage all the relations related to a field, simply select a field from the ones at the top of the dialog. The lower part of the dialog then shows the selected field and the relations related to that field: the ones shown on the left are the links from something (either a table's field or a sequence) and the ones on the right represent links from the selected field to the table's field shown.
Below is a screen shot of the properties dialog of the 'order_contents' table. As the order_id field is selected in the upper part of the dialog, the lower part shows the relation in which this field is engaged (a relation from the 'orders' table and 'id' field to this selected field).