Overlaying is when you modify existing code by changing the system behaviour.
Right click on the object and click Customize. This will add it to your project.
You will see a little [c] to indicate it is a customised object.
I added a bit of code to the initValue method. Notice how it has changed colour to indicate it has been customised.
Extensions allow you to leave the system behaviour but adding your piece to it.
On the CustTable right click and Opern designer.
Right click and copy the vent handler method (this will copy into clipboard).
Now create a class. Paste whats in clipboard. It will paste the highlighted section below.
Add your code. The vent will now be triggered after the initValue method.