UseCases - UML subject vs SysML Block

The first common misunderstanding is that just because a UseCase symbol appears inside the rectangle of a subject Classifier does not mean that it is owned by that Classifier and can only have that one Classifier as subject:

UML-2.5.1: 'UseCases need not be owned by their subject.'

UML-2.5.1: 'A UseCase may be owned either by a Package or by a Classifier. Although the owning Classifier typically represents a subject to which the owned UseCases apply, this is not necessarily the case ...'

UML-2.5.1: 'Note also that the subject rectangle does not imply that the subject classifier owns the contained UseCases, but merely that the UseCases apply to that classifier.'

Some case are explored here:

SysMLv1: MagicDraw/Cameo: INTRINSIC GOTCHA: Care must be taken with ownership of Activities used as UseCase scenarios vs the ‘subject’ of the UseCase. Make sure you don’t get an Activity ‘context’ mismatch! It's up to you to manage it as intended.

The second common misconception concerns the notation itself. A Classifier (such as a SysML Block) acting as a subject may different on a UseCase Diagram than in other diagrams:

UML-2.5.1: 'A subject for a set of UseCases (sometimes called a system boundary) may be shown as a rectangle with its name in the top-left corner, with the UseCase ellipses visually located inside this rectangle.'

UML-2.5.1: 'Note that this notation for the subject classifier differs from the normal Classifier notation – it has no header or compartments.'

Up next

Package, Model, or SysML Block as UseCase owner

Notes

[GOTCHA, TOOL, WARNING] Webel: SysMLv1/UML: MagicDraw/Cameo: AVOID the the vendor-specific «useCaseModel» stereotype for Model packages, it leads to confusion with the rectangular 'subject' notation

[ASSERTION] Visual containment of the symbol for a UseCase in the rectangular symbol for a subject Classifier does NOT imply ownership! Packaging (ownership) of UseCases is separate from use within subjects, and a single UseCase may be used in more than one subject.

[CAPABILITY, DISPLAY, STYLE] In MagicDraw/Cameo the name and any stereotype keywords of a Classifier subject of a UseCase may be shown in the top-middle (not just top-left)

[CONVENTION, MODELLING, TIP]{RECOMMENDED} Webel: MBSE: SysMLv1: Prefer a custom «actor» extension of a Block (such as the non-normative External) over UML-style Actor for use as parts in IBDs and on allocation swimlanes. You can also have a Trace from a block-based «actor» to a UML-style Actor.

Snippets (quotes/extracts)

[UML-2.5.1] UseCase::subject : Classifier [0..*] ... The subjects to which this UseCase applies. Each subject or its parts realize all the UseCases that apply to it.

[UML-2.5.1] A subject for a set of UseCases (sometimes called a system boundary) may be shown as a rectangle with its name in the top-left corner, with the UseCase ellipses visually located inside this rectangle.