Glossary

Every term used in these docs, in alphabetical order. Link here whenever a page uses a term before its home chapter has explained it.

Activity

One step in an F4 flow — something an actor or system part does, named verb-first ("Validate claim"). Lives in a lane.

Admin

A team role with permission to manage members and invitations alongside the owner (on Pro cloud projects). Admins cannot delete the project or change its tier.

Boundary / Group

A labelled container drawn around a set of elements in a C4 diagram, visually grouping them and reducing clutter. A subject's boundary is drawn automatically; other groups are created manually.

Boundary event

A marker on an activity for something that can interrupt or branch it while it runs — a timeout, an error, a cancellation — with its own outgoing path.

C4

A way to describe a software system's structure at four zoom levels — Context, Container, Component, Code. See The C4 model.

Call connection

An F4 connection kind for a synchronous ask-and-answer: the target does its work, answers (optionally named by a return label), and the caller continues to its own next step. Drawn as one connection with a solid request arrow and a dashed return arrow — no separate way-back connection. The default kind, a flow, hands control over instead.

Component

The third C4 level: a structural building block inside a single container — a service, module, or adapter. Also one of the five fixed element types.

Connection

In deployment and F4 diagrams: a labeled link showing that two things communicate (infrastructure traffic, or flow order). The C4 equivalent is a relationship.

Container

The second C4 level: a separately runnable or deployable unit — a web app, an API, a database, a message broker. Also one of the five fixed element types. Not a shipping container or a Docker container, though a Docker container often is one.

Container instance

A deployed, running copy of a container, placed on a deployment node. One container often runs as several instances.

Context

The first C4 level: the whole system as one box, surrounded by who and what uses it.

Deployment diagram

The view that answers "what runs where": deployment nodes holding container instances, per environment. See The deployment model.

Deployment node

A piece of infrastructure something runs on — a region, cluster, host, or managed service. Nodes nest to show containment.

Demo project

The built-in example project (structure + deployment + flows) loadable from the New Project dialog. Used by every guide in these docs.

Element

Any box on a C4 canvas. Carries an element type, a name, and a description.

Element type

One of the five fixed meanings a C4 element can have: Person, Software System, External System, Container, Component. Types can be recolored but not renamed — the type is the meaning.

Environment

A named deployment context — development, staging, production. Each environment gets its own deployment view because topologies differ.

External System

An element type: a system your system depends on but someone else owns. Drawn opaque — its insides are not your model's business.

Export theme

A saved appearance for diagram exports — colors, fonts, and logo placement that you configure once and apply to all PNG/SVG renders.

F4 flow

A swimlane diagram showing how work moves through the system: lanes, activities, hand-offs. See The F4 flow model.

Hand-off

A flow connection crossing from one lane to another — work changing hands. The dynamic counterpart of a C4 relationship.

Lane

A horizontal band in an F4 flow owned by one actor or system part. Lanes can be bound to C4 elements and can nest.

Layer

One canvas in the C4 editor, holding the elements of one zoom level (e.g. the context view, or the containers of one system). Drill-down navigates between layers.

Linked instance

A container instance in the deployment diagram that reuses the structure of a parent instance — the same topology and nested instances, but scoped to a different deployment node or environment.

Person

An element type: a human role that uses the system — "Customer", "Claims Handler". Roles, never individuals.

Process

A named piece of business behavior ("Handle a claim") described by one or more F4 flows.

Project

Everything you've modeled — the C4 structure, deployment views, and flows together. Lives in your browser by default; named projects and cloud sync are available. See Projects & sharing.

Promote

To move a local project to the cloud and enable team sharing (Pro feature). Once promoted, the project syncs to all browsers and team members with access.

Relationship

A directed, labeled arrow between two C4 elements, carrying a purpose (verb phrase) and optionally a technology. Direction = who initiates.

Section

A named horizontal band in an F4 flow grouping related lanes — by phase, theme, or team. Each section can be read at its own zoom level.

Save state

The status of a cloud project's sync: saved (everything in sync), dirty (you have unsaved edits), saving (pushing changes), saveFailed (something went wrong, with options to retry or keep as local), remoteNewer (another browser pushed a change — click Load latest to restore this device, or Review & save to resolve if you have local edits).

Software System

An element type: the thing you're describing, or another whole system yours talks to.

Subject

The single element a C4 diagram is "about" — drawn as the boundary containing its direct children, with a drill-up button to the parent layer. Subjects anchor the diagram's meaning and are used by the wand to populate related content.

Tag

A label on flow activities and lanes ("happy-path", "error-handling") used by the viewer's visibility panel to spotlight or hide parts of a flow.

Update from code

To re-generate a known project from its code extractor and review the changes (adds, edits, removals) before they land. Conflicts (you and the code both renamed the same thing) are flagged for your pick. Located on the project's detail page.

Wand / Scaffold

An auto-generation tool that populates a C4 diagram from a given subject: adds the subject's direct children inside its boundary, and rolls up related elements from across the structure. Right-click the canvas and choose "Wand: this diagram".

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