The Optional Clauses tab lets you define which WHERE clauses will be presented as optional to the user. For example, in the report below, you may want to make the Approval Status parameter optional.

START WITH Document
SELECT
  Id,
  Code,
  Name
WHERE
  IsTemplate = 'N'
  AND ApprovalStatus = @ApprovalStatus

When the report is run, the user is prompted to enter data, and the Approval field is displayed with a check box to let the user include or exclude that criterion from the search.