Because inspectors have misread requirements and making demands that the TC didn't intend. So, at times for clarification, they will say "shall be permitted". These are my personal opinions and are not official interpretation. 

Al, you are correct.  No one requirement, you can choose from the list. Wassamata, you don't like freedom?  ;-}

The stilted language is an NFPA thing - I don't think you'll find anyone who really likes to write code that way, but back in the Burton Klein days that was the rule. We don't do it so much anymore as we have other workarounds, but it still happens. 

OK great news / I am reading this too literally.

Per happens its time the equipment manufacturer 

state within their submittals documents the requirement for 

their NFPA 99 compliance system the type of material

for that system for the Exhaust Lines. 

NFPA 99 the 2024 Edition & Others in Section #5.1.3.7.1.1 (5&6).

The use of brass, galvanized, or black steel pipe is permitted between the source valve and exhaust lines.

As a whole it's up to and determined by the equipment manufacturer the material on the package itself. If those materials are ok on the package. Why not on the exhaust lines?

NFPA 99 / The 2012 Edition:

Section #5.1.3.7.7 Medical-Surgical Vacuum Source Exhaust

This year’s edition has (NO) listing of the material or joining of the material.