I'm not familiar with "RS-18" or "RS-17, however ANSI A17.1, section 2.27.3.2.3, uses the term "Designated Level" and in section 1.3 defines the designated level as:
Designated Level: the main floor or other floor level that best serves the needs of emergency personnel for firefighting or rescue purposes identified by the building code or fire authority
NFPA 72, section 6.16.312.1, "Designated Level Recall" uses the term "...designated level..." and 1.16.3.12.2 "Alternate Level Recall" uses the term "...alternate level..."
The IBC and IFC, refer back to ANSI A17.1. For existing buildings, the standard is ANSI A17.3.
I believe ANSIs' definition put's the responsibility on the local fire inspector to decide, however the norm is what ever level the fire truck rolls up to and walks in. This has been an issue for building that are built on the side of a hill, where the fire department enters on the 2nd, 3rd, or higher floor, because that's where they have a large enough driveway to pull their truck up to.
Keep in mind that one of the purposes of the "Recall" is to bring the elevator to a floor where the fire department can use it to carry their gear up to the emergency.
In 2003 the NEC Digest had a good article titled "Interfacing Elevators and Fire Alarm Systems". In the "March/April" 2006 Issue of the "NFPA Journal" another good article titled "Elevator Controls" was published. The NEC Digest is no longer published and you'd have to be a member of NFPA to view the Journal article.
Not sure if anyone's interested in me posting them, but if so let me know.