I am writing a quick guide on the Set/Parameter Actions pane and what each part does.
Set Actions
Source Sheet - Where am I Clicking?
Target Set - What am I Changing?
Running the action will - What do you want me to change?
Run Action - How do you want me to change mechanically
Parameter Actions
Source Sheet - What am I Clicking?
Target Parameter - What am I Changing?
Source Field - What am I Changing it With?
Run Action on - How do you want me to do this?
Hopefully this brief guide helps you gain a better understanding what each part of the Set Action and Parameter Action does.