What signifies that movements are controlled by the Manual Block System (MBS)?

The Manual Block System (MBS) is a method of train control that relies on the authority of a Rail Traffic Controller (RTC) and is governed by specific block limit signals. In this system, movements are not automated; instead, they require explicit permission from the RTC, who oversees the safe movement of trains within designated blocks. The block limit signals are an essential part of this system, indicating to conductors when it is safe to proceed based on the occupancy status of the track ahead. This ensures that trains are spaced adequately to prevent collisions and to manage the flow of traffic within the rail network.

In contrast to the incorrect options, the MBS does not operate based on automation or mean that movements can occur without RTC oversight. It is not simply governed by speed limits, as that would fail to address the key safety mechanisms in place for controlling train movements within a block system.