3 basic building blocks of UML - Relationships
Structural relationship that describes a set of links, a link being a connection between objects.
a change to one thing (independent) may affect the semantics of the other thing (dependent).
(direction, label are optional)
variants: aggregation & composition
a specialized element (the child) is more specific the generalized element.
one element guarantees to carry out what is expected by the other element.
(e.g, interfaces and classes/components; use cases and collaborations)