Germany has a "historic" registration scheme for vehicles 30+ years old. These need to be in more-or-less stock spec and in good condition to be eligible for certification. Period mods are allowed to a certain degree. While the insurance rates are still calculated on type and size/engine capacity/engine power as all others, road tax becomes a flat rate for all. This is especially advantageous for owners of large engine capacity vehicles, 7l V8s and the kind. Means wifeys' 500 Fiat road tax costs the same as a fuckin' Corvette, which is more than it would cost if the Fiat were "normally" registered! Only real advantage of the "H" registration for the Fiat would be allowance into environmental zones from which non-complient vehicles would otherwise be banned and lower insurance rates under the "H" scheme. No limit on mileage or days of use. Everyday use is discouraged but I've not heard of anybody getting nicked for that.
Insurance rates are generally quite reasonable for historic vehicles, often based on an agreed-limit annual mileage.
Then there's the red "collectors plate" that can cover an entire vehicle fleet. All vehicles must comply to their specific conditions and have been checked and thus certified. The plate can be swapped from one vehicle to the next with its details filled into a log book that is to be carried on the ride. Insurance rates for a historic fleet can often be haggled over, often based only on the most expensive vehicle. This scheme is meant especially for rally attendances or road testing, everday use is not allowed. Doesn't mean it doesn't happen though... Biggest disadvantage is that only one vehicle at a given time can be used. Want to ride out with the boys on parts of your collection? Multiple plates and insurance required.
Then we have the limited rego scheme for everyday vehicles, ie, rego only for the months April-October, which is the most common for bikes that are laid up during winter. Any period can be agreed upon, many classic car owners have a winter hack, accordingly registered. The vehicle can be used for the agreed period only, annual insurance and tax rates are reduced accordingly, rego is suspended for the remainder. Bikers (or Porsche, Ferarri, etc. drivers) that wanted or needed to save cash were previously required to de-register and re-register their bikes every year, with all the attached hassles. The limited period scheme has made this an absolute breeze. The allowed period of use is stated on the rego plate.
Generally, almost all are catered for in Germany and mostly, pretty fairly.

GTL rego plate. Limited registration period April-October, TÜV expires May 2028. SU → Administrative region Rhein-Sieg Kreis (Rhine-Sieg region, the Sieg river being a major tributary to the Rhine) in the federal state of North-Rhine Westphalia. The code for the administration region is usually the first letter of the major city in that region, ie D=Düsseldorf, E= Essen, M=Munich, S=Stuttgart, etc. These can be up to 3 letters. In this case, SU=Siegburg, a major town on the river Sieg. Because all possible letter/digit combos have been used up in many major regions, smaller regions/codes that once fell victim to slimlining these regions are popping up again, proving that the growth in vehicle numbers was not anticipated 50-odd years ago!

Fiat historic rego.
(yes, I know the plate is at an angle... will be rectified when current resto is finalised!)
Hope that clarifies things for those that have always wondered what german plates actually denote. Registration, test and tax certificate, all in one. Not really complicated, just efficient.
piet