Function field_name_is_valid_for_class 

pub fn field_name_is_valid_for_class(field_name: &str, class_name: &str) -> bool

Additionally refuses the default columns, which a client cannot redefine.

Both tables are consulted, _Default and the class’s own (SchemaController.js:474-479). The second is what makes name un-addable on _Role while the same name is ordinary on any other class, and it only reaches as far as default_columns_for models: a class absent from that table has no class-specific columns to protect, so _Installation’s field localeIdentifier cannot be added is not reachable yet.