Relation display options that are applied to all Relations in a Diagram are surfaced as Diagram options. Relation display options that can vary on a per-Relation basis are surfaced as Relation Graphics options. For example, the Line Style option (Diagonal vs Orthogonal) is a Diagram Display option, since you would most likely want to have all Relations in a Diagram drawn one way or the other. Whereas the Line Color option is a Relation Graphic option, since you may want to give different Relations different colors.
You can view the Diagram Display options by selecting a Diagram in the Model Explorer and then selecting the 
tab in the Property Browser and then selecting Relation in the Display Type control. Or you can right click on the background of a Diagram and select Display Relation->Properties from the context menu. Since a Diagram can inherit its properties from a Diagram Template, you can use the <default> Diagram Template to change display options for all Diagrams (that inherit from the <default> Template and haven't overridden the property - see Templates).
You can view the Relation Graphics options by selecting a Relation in the Diagram and then selecting the 
tab in the Property Browser. Since a Relation Graphic object inherits from Relation Graphics Template, you can edit the Default Relation Graphics Object to affect the display of all Relations in all Diagrams (if the property hasn't been overridden).