What does the term 'BULLETIN ORDER (BO)' refer to?

The term 'BULLETIN ORDER (BO)' refers specifically to a temporary order that affects the movement of trains. It is used by railway authorities to communicate important information regarding changes or updates that need to be implemented immediately or within a set timeframe. These orders can include adjustments for train speed, track conditions, or other operational instructions crucial for maintaining safety and efficiency in train movements.

Temporary orders are distinct from permanent changes in operating rules or notices that have no direct impact on train operations, as they are designed to address immediate situations that require prompt action. This is why the correct answer highlights the temporary nature of a Bulletin Order in relation to train movement, emphasizing the necessity for engineers and train personnel to be aware of such orders to ensure compliance and safety in their operations.