UML14 — Use Case Metamodel - UML2 Superstructure issue

I tried to understand some parts of the Use Case metamodel but get
stucked ...

Looking at figure 10-49 (p. 468) of the current (i.d. 3rd rev)
Superstructure document it is not clear or at least not that obvious
to
me how the Actor is related to the Use Case.

The only possibility seems to be the relationship where the UseCase
participates taking the role useCase connected to the Classifier. But
I
don't think that the Actor should play the role subject ...

Further, the relationship connecting Actors with UseCases allows the
placement of Multiplicities but users are not encouraged to use roles.
Why is this asymmetry introduced? I could imagine situations
(especially
for business models) where roles would perfectly make sense even for
Actors. This would be the case always when a actor acts on behalf of
another entity or an actor is to be specialized w.r.t. a specific
context.

Any ideas?

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