Спасибо за ответ.
Подтверждаю и расширяю ответ цитатой из спецификации:
-> UML Superstructure Specification v2.1.2, стр.41:
"Navigability means instances participating in links at runtime (instances of an association) can be accessed efficiently from instances participating in links at the other ends of the association. The precise mechanism by which such access is achieved is implementation specific. If an end is not navigable, access from the other ends may or may not be possible, and if it is, it might not be efficient. Note that tools operating on UML models are not prevented from navigating associations from non-navigable ends. ".