CNAME vs. A Record
For most sites, creating a CNAME is not an issue. In a few cases, it is not possible for a website to use a CNAME. Some examples are if you are running a mail server at that domain or if you have subdomains of that domain. If you think this might be the case, please check with your department IT staff. Otherwise, Hostmaster will notify you if there are any problems when you submit your offsite hosting request.
This page explains our recommendations on how to proceed in this situation: Pantheon A Record Best Practices.