RtlInitializeSidEx - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
#if (PHNT_VERSION >= PHNT_THRESHOLD)

NTSYSAPI
NTSTATUS
NTAPI
RtlInitializeSidEx(
    _Out_writes_bytes_(SECURITY_SID_SIZE(SubAuthorityCount)) PSID Sid,
    _In_ PSID_IDENTIFIER_AUTHORITY IdentifierAuthority,
    _In_ UCHAR SubAuthorityCount,
    ...
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit.