Backend user This group is used to gives user backend access. While users in `base.group_user` gets a lot of default access which makes hard to define properly records/rules/menu access. So for maintainability you shouldn't linked any access right, rules, menu, and so on to this group directly. The only intent of this groups is to be able to get a session to Odoo backend (`/web`). Backend UI user This group is used to gives user basic ui access.