How do you expand a Context Diagram into detailed Use Case Diagrams?
You zoom in one level at a time: each actor's single arrow in the context diagram becomes a cluster of concrete use cases, and any complex use case is broken out into its own sub-diagram.
The context diagram only says who uses the system. To turn that into something you can build, you progressively decompose it:
- Context diagram — the system plus its actors (high-level).
- Main use-case diagram — for each actor, replace their one connection with the specific things they actually do.
- Sub-diagrams — when a single use case is itself complex, give it its own diagram showing the variants underneath it.
Worked example — the mobile payment app. The customer's single "uses the payment" arrow expands into concrete use cases such as pay, load money onto the account, and use extra services. Some of those then explode further into sub-diagrams:
- Ways to fund the account: prepaid, credit card, debit card.
- Ways to pay: NFC tap, scan a QR code, or peer-to-peer (phone-to-phone).
- Extra services: loyalty points, gift vouchers, couponing, and a digital stamp card.
Decomposition also surfaces actors the context diagram glossed over. Beyond the customer, this system really involves a separate end user, a voucher seller, the merchant/seller, a credit-card provider (authorizes card payments), and the bank (authorizes the transaction).
The takeaway is the technique, not this app: keep refining each node until the use cases are concrete enough to write descriptions and test against.