Parameter Control Decks are used for setting Parameter values when a Parameter has been created in the Parameters Pane and its Parameter Control Deck is shown.
In Parameters, a convenient feature was created to Link with Selected Datasources. This feature is enabled when you have a parameter of the same name and type in more than one datasource. When you link a normal parameter with multiple datasources, they will remain linked until the user unlinked them manually.
Keep in mind that for parameters that are part of a query, the link is NOT permanent, you will have to execute the "Link with Selected Datasources" action every time you want to generate the modified parameter value to other datasources.
See the Parameters page for more on how to create and modify Parameters.