区别：
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。
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。
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：