Understanding a combo box filling another table's field

I created a combo box on a ZIP Code table within a
Contact Information form. I used the wizard to set it up
so that when I select a ZIP Code from the combo box
(which displays ZIP Code, City, State from the ZIP Code
table), the combo box stores the ZIP code in the Contact
Information field.

I've looked for code to see how this happens, but can
find none nor is there anything in the Property box that
would make this happen.

Can anyone explain the mechanism here?

Thanks.