Lesson 3 of 3
What a collapsed Pool is
A collapsed Pool is an external participant represented as a plain rectangle with no internal elements visible. It shows that the participant exists and interacts with your process — but their internal process is either unknown or irrelevant to this diagram.
The collapsed Pool communicates via Message Flow to and from your own Pool. Its internal steps are not shown — only the messages crossing its boundary.
When to use a collapsed Pool
- The participant is external to your organisation — a customer, a supplier, a payment gateway, a regulatory body.
- You do not know or do not need to model their internal process. Your diagram is about your process; their internals are out of scope.
- Showing their internal steps would make the diagram too complex — and that complexity would not serve the audience.
Example: In an invoice approval process, the Supplier's internal process (how they generate the invoice, track it, credit-check the customer) is irrelevant. You only care about what they send you and what you send them in return. The Supplier appears as a collapsed Pool, with a Message Flow in (invoice received) and a Message Flow out (rejection notice sent, if applicable).
When to expand a Pool
Expand a Pool when the other participant's internal process is what you are modelling. If a client asks you to document the supplier's fulfilment process in detail, the Supplier Pool must be expanded. If a project requires you to map both sides of a customer-company interaction, both Pools are expanded with their lanes and tasks.
When in doubt, start collapsed
Start with a collapsed Pool for any external participant. You can always expand it later if the internal steps turn out to be relevant. Starting collapsed forces you to focus on your own organisation's process first — which is almost always the correct starting point.
✓ When to use
- External participants whose internal process is out of scope
- Suppliers, customers, regulators, or external systems that communicate via messages
- Starting point for any external participant — expand only when needed
✗ When not to use
- ✕Don't collapse a Pool just to simplify if the internal steps are relevant
- ✕Don't use a collapsed Pool for internal departments — those should be Lanes
- ✕Don't omit external participants entirely — even collapsed, they show cross-organisational communication exists