Service Studio provides a special selector window where you can select the variable you want to use, either in an Assign element of an action flow or in a widget property.
The Select Variable window presents all the variables that are available in the scope of your action or screen, organized into the folders.
Contains all of the variables that are only available in the local scope of your action or screen. These include input parameters, output parameters, execute action output parameters, local variables, and List runtime properties.
Contains all of the widgets you have created in the screen where you are selecting the variable to handle so you can access its runtime properties. Thus, this folder is only available in screens, screen preparations and screen actions.
Contains all of the user defined session variables. See Session variables.
Contains all of the user defined site properties. See Site properties.
Contains all of the Timers you've created, because you can change the value of the Read / Write Timer Runtime Properties.
When used in an Assign, the Select Variable window shows the elements that have already been assigned with an assign overlay.
About Variables | Assign Values to Variables