Webel: SysML: SE: «blackbox»: The custom Stereotype keyword «scenario» indicates a Behavior (Interaction as Sequence Diagram or Activity) that Refines a top-level UseCase within the 'problem' zone.
Webel: SysMLv1: Functional analysis (isolation of white-box Activities identified via «blackbox» scenario Activities of UseCases). Recommend custom stereotype them. Candidate: «whitebox» (or a recommended SE methodology stereotype).
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.
SysMLv1: MagicDraw/Cameo: Synchronising ItemFlows on Connectors with ObjectFlows in Activities using the Item Flow Manager. Nice!
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.
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
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.