I've had a number of questions about the #c4model and notation recently, based upon some of my example diagrams ... for example, "when should I use a red arrow?" and "what do the shapes mean?". The short answer is, "it's up to you". The longer answer is...

The longer answer is the C4 model doesn't prescribe any particular notation -> c4model.com/#notation ... although I do recommend that elements are appropriately labelled to avoid confusion, with colours and shapes being used to *supplement* a notation that already makes sense.

In other words, if you take away all shapes and colours from a diagram, that diagram should still be understandable. So the shapes and colours add additional levels of information, or simply make the diagrams more aesthetically pleasing.

The key to making this work is ... well ... a key/legend. Like real-world maps, this allows you to create a mostly standalone, self-describing artefact. Most groups create their own notations during my workshops, and they're usually all comprehensible.

When it comes to tooling, and specifically @structurizr, I think people are surprised when their diagrams are rendered as a collection of grey boxes by default ... i.e. no shapes and no colours. With this in mind, I'm going to roll out a change to the default diagram styling.

Given our industry's history and people's expectations about what software architecture diagrams should look like, and this is still a work in progress, I'm proposing this as the new default...