UI5
UI5使用Association和Aggregation描述控件之间的关系。
Aggregation：parent和子控件在lifecycle上存在依赖关系：
When a ManagedObject is destroyed, all aggregated objects are destroyed as well and the object itself is removed from its parent. That is, aggregations won't contain destroyed objects or null/undefined.
比如UI5的转盘控件Carousel: 一旦转盘被析构，里面显示的page当然也没有继续存在的意义了，需要跟着被析构。
而Association描述了在lifecycle层面的一种soft dependency关系:
Managed associations also form a relationship between objects, but they don't define a lifecycle for the associated objects. They even can 'break' in the sense that an associated object might have been destroyed already although it is still referenced in an association.
最明显的例子就是控件和其label的关系，比如button和label：技术上来说，可以彼此分开独立存在。
CRM
CRM的Genil model存在三种类型的relation，可以在doman CRM_RELATION_KIND里查看：
区别：
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Association: Link between any kind of objects. Can also be defined across components with root or access object as target. 依赖关系最为loose的一种relation，可以用来连接跨model之间的节点。
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Aggregation: Binds child objects to a root object. Only access and dependent objects can be aggregated. 只适用于同一模型的节点之间的关联。
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Composition: Like an aggregation, but composed child objects always exist. 特殊类型的Aggregation。目标节点的Cardinality为1或者1..n
S/4HANA
只有两种：association或者composition。Composition的含义同CRM里的aggregation，而association的含义同UI5和CRM中的association一致。
C4C
只有两种: association或者composition。C4C的这两种relation多了一个限制：relation的目标BO必须和源BO在同一个部署单元Deployment Unit，或者目标BO位于Foundation部署单元内。
Association的语法如下图：
association的multiplicity只支持[0,1]或者[1,1], 如果不显式指定，默认为[1,1].
C4C relation的一个特色是，一旦申明了一个子节点之后，系统会默认生成一个对应的composition。
下图第4行代码会自动生成一个[0,n]的从root节点到Item节点的composition。
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