is there any difference between using default and 3a 3d in plsql

1DECLARE
2    blood_type CHAR DEFAULT 'O';         -- Same as blood_type CHAR := 'O';
3    hours_worked    INTEGER DEFAULT 40;  -- Typical value
4    employee_count  INTEGER := 0;        -- No typical value
5
6  BEGIN
7    NULL;
8  END;
9  /
10