Web Blocks are reusable screen parts that have a private scope. When a screen uses a web block it can only send information through input parameters, and can't access directly any variables or actions of the web block.
But sometimes you need to send information from the web block to the page where it is running.
An example is when you have a form encapsulated on a web block. When the end-user submits the form, the web block needs to let its parent screen know about the change. The screen can then react to this change, for instance by updating the header, menu, or title.
To make a web block communicate to its parent, and notify that something meaningful has happened, use the Notify action.
In this example the:
Finally the Update Screen action ends, and the web block Save action continues to run.
About Web Blocks | Notify Action