Did u mean QuickTestPro (QTP) if I am not wrong, Reuasble action and external action are part of QTP and has got nothing to do with manual testing.
Reusable Action :– It is something like regression teting because whenever the applicaion comback to the tester after first or more test tester again go for the regression+retesting that is called as the Reusable action. It is an action that can be called multiple times by the test with which it is stored as well as by other tests.external action a reusable action stored with another test.
External Action:–It is something like the monkey testing just for getting to know the flow of the application. It is a read-only in the calling test but u can choose to use a local editable copy of the Data Table information for the external action.