The levels and operation modes are fundamental concepts of the ETCS system (European Train Control System).
European Train Control System - Wikipedia
European Train Control System - Wikipedia
ERTMS Workshop
ERTMS, what you need to know
Operating rules pose a major challenge to ETCS implementation
ERTMS, what you need to know
The first European Train Control System in Hungary
ETCS
ETCS
State of play - European Commission
European Rail Traffic Management System, what you need to know
ETCS Levels and Modes - European Commission
Towards the Single European Railway Area - Infrata
ERTMS - ERTMS
ERTMS level 1 and level 2 Download Scientific Diagram