UML Use Case Diagram

About use case diagrams

A use case diagram answers one question every requirements engagement starts with: what does this system do, and for whom? Introduced by Ivar Jacobson in 1992 and folded into UML 2.5.1 §18, it's the most-taught UML diagram and the lingua franca between product, engineering, and QA. The notation is small — actor stick figures, use-case ellipses, a system boundary, and four line types — but it pins down system scope better than any boxes-and-arrows flowchart.

Schematex implements the UML 2.5.1 visual subset with a single-word usecase keyword and a text DSL built for LLM generation — the PlantUML developer experience without the JVM and without the bracket gymnastics. Distinct from state (intra-object behavior, not system scope), flowchart (no actor / subject / include-extend semantics), and bpmn (how a process executes, not what a system offers).

1. Your first diagram

Every document starts with the usecase keyword, then a header, then declarations and relationships:

usecase
system: "Library"

actor: Member
usecase: "Borrow Book" as Borrow

Member -- Borrow

actor: declares an actor (a stick figure), usecase: declares a use case (an ellipse), and Member -- Borrow draws a plain association between them. system: wraps the use cases in a labelled subject rectangle. The primary actor is placed on the left; supporting actors on the right.

The header accepts:

• title: "…" — a heading drawn above the diagram.
• system: "…" — the subject (system boundary) name. Omit it to let the use cases float free (Schematex warns if you omit it with ≥3 use cases).
• direction: LR | TB — default LR (actors flank the subject horizontally).
• generalization: tree | individual — whether to merge sibling generalization arrows (default tree).

2. Actors

actor: Customer
actor: "Payment Gateway" as PG (external)
actor: "Warehouse Staff" as WH
actor: Admin (left)

• A bare or "quoted" name becomes the label; as ID gives it a short identifier for relationships.
• (external) (or (system)) renders the actor as a rectangle with an «actor» stereotype instead of a stick figure — use it for other software systems (payment gateways, third-party APIs).
• (business) adds the Bittner & Spence diagonal slash across the stick figure.
• (left) / (right) pins the actor to a side. By default the first actor goes left and the rest go right.

Custom stereotypes go in guillemets after the declaration: actor: "Audit Service" as Audit (external) «system». The parser also accepts ASCII <<system>> and normalises it to «system».

3. Use cases

usecase: "Checkout" as Checkout {
  extension point: payment failed
  extension point: stock depleted
}

A use case is an ellipse sized to fit its text. The optional { … } block lists extension points in a compartment below the name, separated by a divider line. Stereotypes work here too: usecase: "Validate Card" as ValidateCard «secured».

4. Relationships

Four line types connect actors and use cases:

Customer -- Checkout
Checkout ..> Pay          : «include»
Pay      ..> ValidateCard : «include»
Cancel   <.. Checkout     : «extend» [payment failed] (extension point: payment failed)

• «include» points toward the included use case — the reusable behavior A always runs. Source includes target.
• «extend» points toward the base — A is the optional behavior, B is the base it extends. You can attach a [condition] and reference one of the base's (extension point: …) entries. (The arrowhead is always drawn toward the base regardless of how you order the endpoints.)
• An «extend» line is drawn in the theme accent color so the rarer, more surprising relationship stands out.

The parser rejects the high-confidence mistakes humans and LLMs both make, with the offending line number: association between two actors or two use cases, include/extend touching an actor, generalization across metaclasses (actor → use case), reused identifiers, and extension-point references that don't exist on the base.

5. Generalization

--|> works between two actors or between two use cases (never across the two — that's a hard error):

actor: User as U
actor: "Premium User" as PU
PU --|> U

usecase: "Pay by Card"   as PayCard
usecase: "Pay by PayPal" as PayPaypal
usecase: "Pay"           as Pay
PayCard   --|> Pay
PayPaypal --|> Pay

The arrow carries a hollow triangle pointing at the parent. When three or more siblings share one parent, the arrows merge into a single shared head (UML 2.5 Figure 18.5 convention); set generalization: individual in the header to keep them separate. Actor hierarchies route as a clean bus on the outer edge of the actor stack.

6. Multiplicity

Quote a multiplicity string immediately beside the endpoint it belongs to:

Customer "1"    -- "*" Checkout
Cashier  "1..*" -- "1" Register

7. PlantUML-style inline form

Coming from PlantUML? The inline declaration form works and mixes freely with the declarative form:

usecase
:Customer: as C
(Browse Catalog) as Browse
(Add to Cart)    as AddCart

C -- Browse
C -- AddCart
Browse ..> AddCart : «include»

:Name: declares an actor, (Name) declares a use case, and as ID aliases either. Schematex is inspired by PlantUML, not a 1:1 transpiler — multiplicity and relationship syntax differ slightly.

8. Grammar (EBNF)

document     = "usecase" NEWLINE header_prop* statement*
header_prop  = ("title:" | "system:") quoted_string
             | "direction:" ("LR" | "TB")
             | "generalization:" ("tree" | "individual")

statement    = actor_decl | usecase_decl | plantuml_inline | relation | note

actor_decl   = "actor" ":" name ("as" IDENT)? actor_kind? stereotype?
actor_kind   = "(" ("external" | "system" | "business" | "left" | "right") ")"
usecase_decl = "usecase" ":" quoted ("as" IDENT)? stereotype? extpoints?
extpoints    = "{" ("extension point:" TEXT)+ "}"
plantuml_inline = ":" name ":" ("as" IDENT)?      ; actor
                | "(" name ")" ("as" IDENT)?       ; use case

relation     = endpoint relop endpoint label_clause?
endpoint     = (IDENT | quoted) multiplicity? | multiplicity? (IDENT | quoted)
relop        = "--" | "-->" | "..>" | "<.." | "--|>"
label_clause = ":" stereotype? condition? extpoint_ref?
stereotype   = "«" TEXT "»" | "<<" TEXT ">>"
condition    = "[" TEXT "]"
extpoint_ref = "(extension point:" TEXT ")"
multiplicity = quoted_string                       ; "1", "*", "0..1", "1..*"

9. Standard compliance

Schematex follows the OMG UML 2.5.1 §18 (UseCases) visual subset and the Bittner & Spence style guide: guillemets (never ASCII <<>>) in the rendered SVG, «include» toward the included use case, «extend» toward the base, hollow-triangle generalization heads, and the «actor» stereotype on external-system rectangles. Textual use-case descriptions (Cockburn fully-dressed form), scenario tables, and XMI export are out of scope for a diagram renderer. See docs/reference/29-USECASE-STANDARD.md for the full specification.