RtlInitializeSid - NtDoc

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

NTSYSAPI
NTSTATUS
NTAPI
RtlInitializeSid(
    _Out_ PSID Sid,
    _In_ PSID_IDENTIFIER_AUTHORITY IdentifierAuthority,
    _In_ UCHAR SubAuthorityCount
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.