When are python sunder names used?

In Python, there are conventions for single leading underscore, double leading underscore, double leading + double trailing underscore, and single trailing underscore. Many of those are outlined in the answers at What is the meaning of a single- and a double-underscore before an object name?.

But what is the meaning or convention for single leading + single trailing underscore? I've first seen their use in the enum module:

8.13.15.3.2. Supported _sunder_ names

_name_ – name of the member

_value_ – value of the member; can be set / modified in new

_missing_ – a lookup function used when a value is not found; may be overridden

_ignore_ – a list of names, either as a list() or a str(), that will not be transformed into members, and will be removed from the final class

_order_ – used in Python 2/3 code to ensure member order is consistent (class attribute, removed during class creation)

_generate_next_value_ – used by the Functional API and by auto to get an appropriate value for an enum member; may be overridden

I have not seen such single-leading-single-trailing underscore sunder names before. Are they treated in any special way, or otherwise have an implied meaning that is distinct from any of the other underscore-related naming conventions? How are they different from having no underscores at all?

They are not treated in any special way. They are being used by the enum module so as to

Not to be accidentally overridden

e.g.

class Status(Enum):
    alive = auto()
    dead = auto()
    missing = auto()

You can see that Status.missing and Status._missing_ are different objects. If _sunder_ names, were allowed, we could accidentally override the _missing_ function which would cause unexpected errors.

not appear as private. The name _value in python is considered private. To express that these are not private (which again, a user might want an enum value to be private), they are instead given sunder names
other alternatives such as __double_leading_underscore and __dunder__ also have special meanings in python as you stated above. _sunder_ methods in Enum act something like the _dunder_ protocols of pure python, but aren't reserved by the language.

Basically, it's an option to avoid attribute name conflicts without giving the wrong impression.