Often very confusing to those not within the industry, it can be difficult at first to understand the differences in these three terms. Briefly:
Fall restraint systems restrict the movement of the worker in reaching the fall edge. The lanyard is typically a fixed length that does not extend past the fall edge and it effectively acts like a leash preventing a fall arrest situation from occurring.
As its name suggests, fall arrest systems arrest the fall of a worker once it has occurred. They do not prevent the worker from falling and have the highest potential for injury. A rescue plan must be in place prior to any person using the fall arrest device.
For a more detailed description: https://sayfa.com.au/fall-arrest-fall-restraint-fall-prevention/