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Hello,
As described in the documentation, you can use the same as class diagram. For example:
@startumlpackage "AAA" as a {object Object01object Object02object Object03object Object04}package "BBB" as b<<Cloud>> {object Object05object Object06object Object07object Object08}Object01 <|-- Object02Object03 *-- Object04Object05 o-- "4" Object06Object07 .. Object08 : some labels@enduml
http://www.plantuml.com/plantuml/png/NOvD2i8m44RtESL7baABFwsAbDBM0tY6IHi4JOoOPANUtLA8WLaDxrkyUPqFz0hH6l6ayKvN3TdtlGHvWF0MJjtq671Ppdh3UCjunxWIt-mTXc7nggOv6nUdjWNtruptZ0-CZxC_fQ7v54KEIqLOvRLAQmqthxAI-L7wWBByst62TrR3aDB6Yqu_fcZD3m00