The logic of your application module is implemented through Actions. Actions are coded into an action flow, where you can use control tools, queries, etc. The flow of an action is defined by using a specific editor.
In OutSystems Platform, you can have:
Screen Actions: Actions associated with a screen that reacts to the feedback from the end-user;
Screen Preparation: Actions executed before the rendering of the screen;
User-defined actions: Actions defined by the end-user available in any flow or expression (in a case where they're defined as functions) of your application module.
Design Actions | Action Flow Editor | Use an Action Reference | Find and Replace Action Usages