Google Cloud’s three role types. Basic roles (Owner, Editor, Viewer) are very broad; predefined roles are maintained by Google with each service’s permissions; custom roles are defined by you with the exact permissions and are created at the organization or a project, not folders.
On the ACE the rule is least privilege: avoid basic roles in production, prefer predefined roles, and create a custom role when none fits (for example, by copying a predefined one with gcloud iam roles copy).
IAM roles: basic, predefined, and custom is a concept that can show up on the exam. Check the full guide to see it in context.