You can use the Expression window to compose expressions to use in expression filters or in calculations. Expressions that you create for expression filters must be Boolean (that is, they must evaluate to true or false).
While you compose expressions for both expression filters and calculations, the end result is different. A calculation becomes a new data element that you can add to your visualization. An expression filter, on the other hand, appears only in the filter bar and can’t be added as a data element to a visualization. You can create an expression filter from a calculation, but you can’t create a calculation from an expression filter. See Create Calculated Data Elements and Build Expression Filters.
Directly enter text and functions in the Expression window.
Add data elements from the Data Elements pane (drag and drop, or double-click).
Add functions from the function panel (drag and drop, or double-click).