A parameter action writes a value when a reader interacts with a mark. Create the parameter first, then use Dashboard > Actions > Change Parameter to choose the source sheets, trigger, source field and target parameter. A calculation, reference line or other part of the view must use that value for the interaction to have a visible purpose.
The source field's aggregation belongs to the action. It can differ from the aggregation displayed in the source chart. With None, a selection containing different source values can't supply one parameter value. An explicit SUM, AVG or another supported aggregation reduces the selected values to one number.
Clearing is a separate decision. Keep current value makes a click persist after deselection. Set value to restores a known default such as All. Test both the selection and the cleared state, since a dashboard can look correct after a click and still confuse its reader when the selection disappears.