Leaf actions\conditions in Behaviour Tree notation communicate over the Blackboard.
It’s a key\value storage global to Behaviour Tree that can be used by any leaf actions\condition.
Each Behaviour Tree has each own Blackboard.
Input\output pins are implemented over Blackboard by using the same name for ports.
Also Blackboard value can be set or read by any leaf function.
Behaviour Tree notation does not define visual notation for data flows diagrams.
Visual notation I’ve published before for data flows between Functional Blocks can be easily mapped to Blackboard objects
When Behaviour Tree is used with atomic functions data flows between functions are modeled by functional exchanges. Input\output pins send\receive not individual data but ExchangeItems that contains individial data that is called ExchangeItem Element.
In other words in case of atomic functions data pins are modeled implicitly by functional exchange pins and their ExchangeItem Elements. It can be interpeted as composite functional pins (typed by ExchangeItem) that contain data pins (ExchangeItem Elements typed by data types).