On my website I maintain a list of radio station class codes. I originally created the list to organize my own logs, but decided to share it with other DXers. I have gone through many renditions over the last decade.
Today I published a major update. I was not happy with my use of modified FCC service codes as they were admittedly confusing. Not only did some conflict with ITU codes, but some also conflicted with FCC codes themselves. I created a lot of new codes - for standardization - which turns out was not the best idea. So now I've gone back to reintroducing the services and codes used by the ITU, including some extra codes for station types listed in the primary ITU station definition page which have no codes assigned to them. The ITU codes included are a combination of Station Class codes, Nature of Service codes and codes from the List of Coast Stations and Special Services publication.
The use of ITU codes has more acceptance among the DX community. However, many ITU codes have ambiguities. The separation of "station class" and "nature of service" is not cast in stone. A good example is the "nature of service" code AX (Aeronautical Fixed), which is used as a "station class" code in the ITU publication Alphabetical List of Call Signs, and in hobbyist publications like the old CFL (Confidential Frequency List).
There also isn't a clear enough distinction between codes such as MX and BM in the ITU definitions; but we understand MX to be intended for synoptic weather broadcasts and BM for marine weather broadcasts. Aeronautical broadcasts are technically lumped under the BM (formerly S7) code, but here we've introduced the VM code - derived from the V used in the Volmet allocation tables. (The FCC/FRC used to have an AW code for these, which stood for "Airways Weather").
The ITU publications give no clear indication of how Noaa Weather Radio broadcasts should be classified. We would put them under the MX code - as they are intended for "general (public) reception", like the PX code (although technically that phrase is missing from the MX code although it appears in the PX), and are not stations that participate in any mobile service. Clearly, the ITU needs to clean up these ambiguities and modernize the definitions and ancient codes that were developed in the 1920s.
Even the ITU services aren't clear. The old "special service" (formerly SP), which once included the now separate standard frequency & time signal and radiodetermination services, continues to live on in ambiguity. It was originally meant for classifying broadcasts that were made by stations other than standard AM or SWBC stations (with oddities like medical advice consultations thrown in). Now these stations are usually classified under the same codes as their mobile counterparts. The "safety service" (FS) is apparently orphaned and not clearly part of the service structure.
I hope to have less changes to my list in the future. It needs some stability. I've just updated my own personal logs to reflect the changes.
The new list can be found here : http://dxinfocentre.com/reference/STC.htm
The old list can be found here : http://dxinfocentre.com/reference/STC_old.htm