[jason taylor]

For an internal messenger, carrying high priority state material like a Queens Messenger, or a Mongol gold tablet carrier. Or the guy carrying the new issue of cords for the really important stuff that can't be carried by cable or radio. Or just your ordinary runner or dispatch rider:

Diplomatic Immunity: Only becomes relevant when carrying messages to a diplomat outside the country. And only when agreed between the countries. SOE agents taking stuff in and out of France for instance don't count for that: if they get caught they go into a Deep Dark Hole (I rather wondered myself at the German government's weird propensity to send trained assassins to Dachau with a lot of desperate potential rebels around them). But in peacetime even someone carrying something meant to go to a spy will be ignored. Certainly the courier will. The Case Officer will probably be sweated for a few hours and released even if his asset goes to a Deep Dark Hole.

Low Level Legal Immunity: You are an exception to traffic law the way emergency vehicles are. No LEGAL advantage if you do a felony (your advantage is that you have an excuse to be elsewhere quick and there is always another agency that wants you to be). Only applies to those who know your ID. If you are in plain clothes you don't get it until you give your password. If it is an esoteric military specialty, only someone with heraldry, Savoire-faire military, etc will give this to you. Or at least they won't until they get a phone call from someone important.

Security Clearance: Probably not enough to know what you are carrying but enough to handle that sort of thing.