Substitutable column clause [Oracle SQL]

[ element ] is of [ type ] (only type-name)

[ not ] substitutable at all levels

create type base_type as object (
  num number,
  dt  date
) not final
/

create type deriv_type under base_type (
  str varchar2(10) 
)
/

create table allows_subtypes (
  id  number,
  bt  base_type
)
column bt substitutable at all levels;

create table does_not_allow_subtypes (
  id  number,
  bt  base_type
)
column bt is of type (only base_type);

begin
  insert into allows_subtypes values (1, base_type(1,sysdate));
  insert into allows_subtypes values (1, deriv_type(1,sysdate, 'x'));
end;
/

begin 
  insert into does_not_allow_subtypes values (1, base_type(1,sysdate));

  -- Next line would cause ORA-00932 if uncommented
  -- insert into does_not_allow_subtypes allows_subtypes values (1, deriv_type(1,sysdate, 'x'));
end;
/

create table unclear (
  id  number,
  bt  base_type
)
column bt element is of type (only base_type);