Coding Accuracy Support System, or CASS certification, is the process of matching street addresses to the USPS database to ensure accuracy and deliverability of the mail. CASS certification also adds carrier route and Zip+4 information that may allow for lower postage rates when presorting your mail.
Here's an example:
Our address is 9777 Green Park Industrial Drive.
It is often mistakenly addressed as
Our CASS software knows these addresses do not match up with our actual address and modifies the address with the Zip +4 extension so it can be sorted properly.