In Service Studio a Variable is a writable element in the scope of your screen or of your action or process flow. It is an element to which you can assign a value.
Possible variables are: screen input parameters, screen and action input parameters and output parameters, execute action output parameters, local variables, Read/Write runtime property elements, session variables and site properties.
Variables are picked from the Select Variable window, but you can also type them directly in the Variable of the Assign element in action flows or in a widget's property.
Select Variable window