Just like UserDict and UserList, UserString is a string wrapper that allows easier subclassing of strings due to providing the underlying string as an attribute. The preferred way to do this is to subclass string directly; this class is provided mainly due to backwards-compatibility or simple cases where subclassing string is overkill for functionality.
While all string methods are available, such as UserDict and UserList, UserString adds the data attribute for easy access to the underlying string object. The contents of UserString are initially set to a copy of some type of sequence; the sequence can be bytes, a string, another UserString or subclass, or any other sequence object that can be converted to a string.