It depends on what your operating system's crypt(3) supports. Most Linux/Unix system should support MD5crypt, but that is no longer considered secure (per the original author, PHK).
Some crypt implementations are non-portable, so using crypt can be problematic. Generally with the SSHA2 contrib module, there's not a need to go the crypt route.
More information at <https://github.com/openldap/openldap/tree/master/contrib/slapd-modules/passwd/sha2>
--Quanah -- Quanah Gibson-Mount