There are 3 types of actions available in QTP:- 1)Non-Reusable 2) Usable and 3) External actions.
Non-Reusable action can only be called once in your test. It can not be called in any other test. Resuable action can be called multiple times in your test as well in other test and when it is called in another test then it becomes external action. External action will be a read only untill you saved it locally in your test