差別：
Association: Link between any kind of objects. Can also be defined across components with root or access object as target. 依賴關系最為loose的一種relation，可以用來連接配接跨model之間的節點。
Aggregation: Binds child objects to a root object. Only access and dependent objects can be aggregated. 隻适用于同一模型的節點之間的關聯。
Composition: Like an aggregation, but composed child objects always exist. 特殊類型的Aggregation。目标節點的Cardinality為1或者1…n
S/4HANA
隻有兩種：association或者composition。Composition的含義同CRM裡的aggregation，而association的含義同UI5和CRM中的association一緻。
C4C
隻有兩種: association或者composition。C4C的這兩種relation多了一個限制：relation的目标BO必須和源BO在同一個部署單元Deployment Unit，或者目标BO位于Foundation部署單元内。
Association的文法如下圖：