They don’t really need each other. You can very well use arc42 with and without UML.
Graphical modelling with UML does by no means make you a better architect, nor does it (automatically) improve your systems.
You might use UML to describe or specify the following aspects of your architecture:
- static structure (building block view)
- runtime behavior or runtime scenarios (runtime view)
- deployment or infrastructure
- static or dynamic aspects of crosscutting concepts can sometimes be expressed in UML models.
See also question B-3 (UML).
Some personal comments (from Gernot)
- Unless absolutely required, challenge the use of (pure) UML
- If you use UML, be pragmatic about it (unless you generate code from models)
- Make “developer acceptance” an important criteria when deciding about documentation approaches.