As mentioned above tick New selection whenever you want to search the entire document base for which you possess rights.
As a rule, when you start up the program all documents appear that you are allowed to see. 'New selection' is the default value in the search dialog box. For workflow implementations it may make sense to display a selection that has already been narrowed down (only those documents are shown, for example, which need to be processed). In such a case please refer to the specific workflow documentation.