SysMLv1: MagicDraw/Cameo: Having a SystemContext as the 'subject' of each main UseCase plays nicely with the feature for automated creation of usage-level allocation swimlanes in SysML Activity Diagrams for part properties. But it's not the only way.

Icon class
icon_class
far fa-sticky-note
icon_class_computed
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Note kind
CONVENTION
FEATURE
MODELLING
OPTION
TIP
TOOL
Policy level
INFORMATIVE
Specification keywords
SysML-1.6
SysML-1.7
UML keywords
UseCase
Use Case scenario
Actor
UseCase::subject
SysMLv1.x keywords
SysML Activity Diagram
swimlane
custom block-based «actor»
part property
SystemContext
Keywords
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For an explanation please visit:
SysMLv1: MagicDraw/Cameo: Automated creation of usage-level allocation swimlanes in SysML Activity Diagrams for part properties of a Block. EXAMPLE: A UseCase scenario within a SystemContext as UseCase ‘subject’.
SysMLv1: UseCase scenario representations: On use of Activity Diagrams with swimlanes and/or Sequence Diagrams (Interactions): Actor or block-based custom «actor» on 1st (typically left) Lifeline/Swimlane, System/Subsystem on 2nd Lifeline/Swimlane
Please be mindful that:
SysML/MBSE: Many videos, tutorial slides, and guides to MBSE with SysML present particular modelling recipes as though they are THE way to do something in SysML, not just ONE way of doing it in SysML. They are not necessarily enforced by the SysML spec.
SysMLv1: A common misunderstanding: 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', and it does not imply ownership!
Visit also:
SysML/MBSE: Cameo/MagicDraw: MagicGrid® and the MagicGrid® Book of Knowledge: A pragmatic review of some selected aspects by Dr Darren of Webel IT Australia

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Relates to
SysML/MBSE: Cameo/MagicDraw: MagicGrid® and the MagicGrid® Book of Knowledge: A pragmatic review of some selected aspects by Dr Darren of Webel IT Australia
Webel SE: pragmatic tool-friendly functional analysis: 2-level drill-down from a single Use Case via a «scenario» Activity with allocation to «logical» subsystems first then crossing to the «design» Blocks zone.
Related notes
[MODELLING, TIP]{INFORMATIVE} SysML/MBSE: Many videos, tutorial slides, and guides to MBSE with SysML present particular modelling recipes as though they are THE way to do something in SysML, not just ONE way of doing it in SysML. They are not necessarily enforced by the SysML spec.
[MODELLING, OPTION, TIP]{INFORMATIVE} SysMLv1: MagicDraw/Cameo: Automated creation of usage-level allocation swimlanes in SysML Activity Diagrams for part properties of a Block. EXAMPLE: A UseCase scenario within a SystemContext as UseCase ‘subject’.
[GOTCHA, MODELLING, OPTION, TIP, WARNING]{INFORMATIVE} 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.
[CONVENTION, MODELLING, OPTION, TIP]{INFORMATIVE} SysMLv1: UseCase scenario representations: On use of Activity Diagrams with swimlanes and/or Sequence Diagrams (Interactions): Actor or block-based custom «actor» on 1st (typically left) Lifeline/Swimlane, System/Subsystem on 2nd Lifeline/Swimlane
[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.
[CONVENTION, MODELLING, TIP] Webel: SysML for MBSE: The frequent recommendation that each UseCase have at least one "primary" scenario is a very useful and highly recommended CONVENTION (only). But it is not actually enforced by the SysML1.7 or UML2.5.1 metamodels or specifications.
Related notes (backlinks)
Related snippets (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.
  [UML-2.5.1] A UseCase may apply to any number of subjects. When a UseCase applies to a subject, it specifies a set of behaviors performed by that subject, which yields an observable result that is of value for Actors or other stakeholders of the 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 that this notation for the subject classifier differs from the normal Classifier notation – it has no header or compartments.
  [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.
Visit also
Figure D.5 - Establishing Top Level Use Cases for the Hybrid SUV
Figure D.6 - Establishing Operational Use Cases for Drive the Vehicle
SysML/MBSE: Cameo/MagicDraw: MagicGrid® and the MagicGrid® Book of Knowledge: A pragmatic review of some selected aspects by Dr Darren of Webel IT Australia
15:01: The Allocate relationship and functional analysis
Section: SysML-1.6 specification diagrams: 15 Allocations
Figure 15-2: Abstract syntax expression for AllocateActivityPartition
UseCases - UML subject vs SysML Block
Package, Model, or SysML Block as UseCase owner
GOTCHA: UML Component «subsystem» vs non-normative SysML «subsystem»
SysML/MBSE: Cameo/MagicDraw: MagicGrid® and the MagicGrid® Book of Knowledge: A pragmatic review of some selected aspects by Dr Darren of Webel IT Australia
Visit also (backlinks)