https://www.commonapp.org/CommonApp/Instructions.aspx
WHAT IS A CEEB/ACT CODE?
In the 'Academics' section, you are required to enter the school you now attend or from which you graduated. The first field that must be completed is the CEEB/ACT Code, and a 'lookup' feature is provided to make the process easier for you as an applicant. Simply click on the 'lookup' link to search for your current school by country and city, then click on your school's name. The pop-up window will close, and your school's CEEB/ACT Code and Location will be pre-populated on the page.
Correct entry of the CEEB/ACT Code is extremely important to the accurate and timely processing of your submitted application.
