As an example, from the video rental model, a Customer borrows movies, I wanted to add one caption on the Customer side, "Borrows", and on the other side, "Borrowed by".
I used before ER/Studio from Embarcadero and you could add descriptions on both ends of a relationship. I find that sometimes having captions on both sides of a relationship can enhance the readability of the model.
Currently this can be achieved with Label Rectangle or Text but it is more work to add them or link them, because they don't move automatically along with the relationship line.