My October 1993 timetable' References and Symbols page describes InterCity Shuttle trains as having:
- First and Standard accommodation
- Reserved seats available
- Frequent and regular service
- Enhanced welcome and access at London Terminal Stations
- At-seat service of food and hot and cold drinks in First Class and Standard accommodation
Whereas InterCity trains have:
- First Class and Standard accommodation
- Light refreshments and hot and cold drinks (available for whole or part of the journey)
- Reserved Seats available
And some trains have an extra symbol to indicate Pullman, Restaurant, or Silver Standard service.
A quick flick through the timetable pages didn't reveal any routes not already mentioned above.