User-defined non-inherited fields can be deleted from a table in ServiceNow. These are fields that are created by users on a specific table and are not inherited from a parent table. Inherited fields cannot be deleted from a table, as they are defined on a parent table and shared by all child tables. Any field on a table cannot be deleted, as some fields are system-defined and essential for the table functionality. Table records are not deleted when a field is deleted, as the field deletion only affects the table structure and not the data. Reference: Delete fields

Roles are used to control access to applications and modules in the application navigator. Each application menu or module has a role field that specifies which role is required to view it. Users who do not have the specified role or a role that contains it will not see the application menu or module. Roles can also be used to control access to other features and capabilities in ServiceNow, such as tables, fields, scripts, and workflows.
References = Enable or disable an application menu or module, Grant a role access to applications and modules, Controlling access to applications and modules.