A retention system is a system which is intended to prevent falls from occuring right from the outset.
The users are kept away from the fall danger zone by adjusting their connectors to a set length to prevent them from accidentally stepping out over the falling edge.
You can set up a retention system simply by attaching the users' personal fall protection equipment to a suitable device. Anchorage points, lifeline systems etc., used in combination with appropriate connectors, are typical examples of a retention system. All the users have to do is to connect their connectors up to their safety harnesses and adjust the length accordingly using a rope shortener to stop them from being able to reach the falling edge.