Use case diagrams (UML)
A use case diagram (UML) shows who uses the system and what they can do, without the “how”. Analysts draw it early because customers understand it easily.
| Actor | A person or external system that interacts, such as the payment gateway. |
| Use case | A goal the actor achieves; named with a verb, drawn as an oval. |
| System boundary | The rectangle: what's inside is part of the system. |
| «include» | Always happens: “Place order” includes “Pay online”. |
| «extend» | Optional, under a condition: “Apply coupon” extends “Place order”. |