The Request Types policy cannot be overriden by project managers. It controls:
Project Process Request Type: The request type associated with the projects using the project type.
The request type determines the fields that appear in the Project Overview page > Details tab. And the workflow associated with the request type drives the overall project process.
Issue Request Type, Risk Request Type, and Scope Change Request Type: The request types used when creating project issues, risks, and scope changes from the Project Overview page.