Deployment Manager relies on the platform import and export capabilities to deploy applications and can handle schema management through the same process. Our recommendation is to let the platform import process, and by extension Deployment Manager, handle schema management ( example introducing new classes/tables, column population for exposed properties). Pega only generates and applies additive schema and does not do any type of destructive schema, so it is safe to stick with this process. For more information on configuring Deployment Manager to manage schema, please follow the instructions on Configuring settings to automatically deploy schema changes.
Sometimes company policy dictates that schema needs to go through a DB change management process in which case, you can take away the privilege to automatically apply the schema and then it will automatically pause the pipeline if schema is detected and email the administrator of the pipeline with the schema attached to the email. For more information read the description of the process in Schema changes in application packages.