The value of aggregate for the aggregation property of an association end signifies a whole-part relationship between two classifiers. An association, with one of its ends specifying a value of aggregate is commonly called an aggregation relationship. It is a stronger than an unadorned association end, adding a concept of a whole (hollow diamond end) part relationship. The whole does not have responsibility for the lifetime of the parts. Only one end of a binary association can be adorned with an aggregation symbol. Formally it is one of three values that can be set for the aggregation property of an association end the others being none and composite.
Explanation
Aggregation is a type of association that adds the concept of one thing being made up of other things. The element on the hollow diamond end represents the whole and the set of elements on the other end are the parts. An element may have just one or many parts.
Important Points
An association can have one (and only one) aggregate.
If one end of an association is adorned with an aggregate symbol the other association end may not have an aggregate adornment. This rule only applies to binary associations as n-ary are not permitted to have aggregate or composite set for the aggregation property. The aggregate symbol means that the classifier attached to the association end is acting as a whole while the classifier (at the opposite end of the association to the aggregate) is considered to be a part of that whole.
A composite is a type of aggregation that is stronger, or that has more responsibility than the weaker aggregation. The composition adornment is used when the whole part (filled diamond end) is responsible for the lifetime of the part or parts (tail end).
An association is the base type of relationship having at most two association ends. An aggregate (or composite) can only be added to a binary association not to an n-ary association. The aggregation adornment is added to one of the two ends, the target. The "vanilla" association indicates no whole and part relationship, which is added in this case by the aggregation property being defined as aggregate.
