What does it mean? It stands for  Coding Accuracy Support System (CASS™) is a certification system used by the United States Postal Service® to ensure the quality of ZIP + 4 coding systems. Adding the zip plus 4 to your mailings will help speed up the delivery of your mail,   CASS also standardizes the mailing to correct any misspellings during data entry or inconsistencies  in the file format.   Address Quality is the result and benefit from using a Post Office™ certified process to clean and standardize your data, by correcting spellings, abbreviations, city, state and ZIP Code™ entries.