A free C4 diagram tool should let you answer a real architecture question before asking you to upgrade. Start with a system boundary and the dependencies people frequently misunderstand, then add detail where it changes a decision.
UXXU’s free plan supports one user and up to 400 architecture items, with unlimited projects and diagrams. The pricing page explains the current allowance, paid seats and taxes. An architecture item is a model element such as a system, actor, application, container, component, store or group; diagrams are not counted as items.
A free-plan modeling budget
For a first checkout model, budget three actors, one main system, four external systems, eight applications or stores and twelve components. That is 28 elements before adding any groups or other supporting elements. A smaller scope is easier to validate than an inventory of every library in the repository.
Reuse the same element in another view when it is the same architectural entity. Do not create a second payment provider just because the first appears in your context diagram. Reuse makes the model consistent; it also avoids needless duplicate elements.
Explore an example with meaningful dependencies
This published e-commerce diagram contains 26 visible elements and 30 relationships, verified on October 1, 2026. These are diagram counts, not the source workspace’s total usage or a claim about its creator’s subscription. Inspect the owned data stores and the broker: can you explain what remains available when notifications are delayed?
Build and verify your first model
Download the smaller checkout model as a portable reference. Its DSL is a learning format, not a UXXU import file. Use the same system names and responsibilities when building your own visual model.
Create a context view first, then a container view. Reuse the payment provider, add the checkout API and order store, and label communication paths with a purpose and protocol. Rename the provider and verify the name in every view that references it. Share the result with someone outside the implementation team and ask them to explain the purchase path.
The point of this exercise is to learn whether the model reduces repeated explanation. Upgrade when item usage or collaboration needs justify it. Start in the UXXU C4 model tool or browse more public examples.