The correct sequence of steps the developer needs to perform is:

Step 1 = Create the ProcessPurchaseOrders.xaml file

Step 2 = Open the ProcessPurchaseOrders.xaml file and create the arguments

Step 3 = Invoke the ProcessPurchaseOrders.xaml file in the Main.xaml file and click Import Arguments

Step 4 = Pass the values of the arguments to/from the variables in the Main.xaml file

This sequence will ensure that the developer can create a reusable workflow, invoke it from the main workflow, and pass data between them using arguments. 

https://forum.uipath.com/t/import-arguments-in-invoke-workflow-file/1923

https://forum.uipath.com/t/pass-arguments-invoke/132595