ICAO Annex 11 and PANS-ATM require some interpretation and some expansion to be deployed operationally. Unlike ATC service, the content is insufficient, particularly in relation to the provision of FIS using surveillance.
The UK FIS provides this extra content but splits up the different ICAO requirements (those related to IFR/VFR and identified and unidentified flights) into different service levels. The idea is that it makes it more intuitive for pilots but it hasn't done that. In part because the training of pilots in air traffic services is woefully inadequate leaving them to read and learn it for themselves. Many pilots have done that and truly understand what is going on but a large minority don't bother which has safety implications for all other airspace users.