- The Frequent Network runs often enough that you don't have to plan your trip around a timetable. That typically means every 15 minutes or better all day, but it needs to be more frequent than that where aiming to serve relatively short trips -- as in the case of downtown shuttles for example. If you aren't willing to plan your life around a bus schedule, you are interested only in the Frequent Network.
- Infrequent All-day services are the rest of the service that runs all day. This network often relies on timed connections.
- Peak-only service exists only during the peak period. It mostly takes the form of long commuter-express routes that add lots of complexity to a system map but represent very specialized services for limited markets.
I just keep reading Jarret Walker, and just keep understanding more. It's like learning to paint from one of the 'Old Masters'.
Also related: The idea of a 'visual heirarchy' of transit lines.