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You can either set the value of a parameter by giving it a value OR by looking up the value in a Result row from a previous transformation. Simply specify a column name in that case.
In Mapping (execute sub-transformation) Step
You can pass named parameter values to sub-transformations in the 'Parameters' tab of the 'Mapping (execute sub-transformations)' step: 
Atention:
As of PDI 4.2.x (?), the named parameter must be defined in the sub-transformation, otherwise the value set here will be ignored.