Suppose we want to pass a File Source name from Parent Flow to a Child Flow. Here are the steps you would take in the Parent flow.
Use SetChildContext action in the Parent flow (as shown below). Double click on its properties:
Activity ----------> Name of the File Source Activity in the Parent Flow.
childActivityName -------> Name of the Child Activity whose attribute (fileName) you want to dynamically assign with the fileName of the Parent Flow's Source File. You can skip this property and can also set a stand alone variable name in the childKey property as mentioned next.
ChildKey -------> variable name that willl contain the value of fileName and will be passed to Child flow. In Child Flow you can then use this variable anywhere in that flow.
ChildName -----> is the name of the Call activity in the Parent Flow which is calling the child flow.
Key --------> is the attribute of Parent's File Source (which you selected in the first property above). The value of this attribute is assigned to either the ChildKey or childActivityName above.
varFileName variable below can now be used in the Child Flow's File Source activity. Use the PutContextVar to assign the value of varFileName to the fileName attribute.
Refer to PutContextVar usage instructions in the Help. Note that the syntax to assign a variable value in the PutContextVar is $$variable name$$