About the sentinel object you can read on the official website :
The sentinel objects should behave as expected by a sentinel object: When compared using the is operator, it should always be considered identical to itself but never to any other object. Creating a sentinel object should be a simple, straightforward one-liner. It should be simple to define as many distinct sentinel values as needed. The sentinel objects should have a clear and short repr. It should be possible to use clear type signatures for sentinels. The sentinel objects should behave correctly after copying, and sentinels should have predictable behavior when pickled and unpickled. Such sentinels should work when using CPython 3.x and PyPy3, and ideally also with other implementations of Python. As simple and straightforward as possible, in implementation and especially in use. Avoid this becoming one more special thing to learn when learning Python. It should be easy to find and use when needed, and obvious enough when reading code that one would normally not feel a need to look up its documentation
Let's see one default example:
from typing import assert_type
MISSING = sentinel('MISSING')
def foo(value: int | MISSING) -> None:
if value is MISSING:
assert_type(value, MISSING)
else:
assert_type(value, int)