When should you line out a red W in communications with an aircraft?

Lining out a red W in communications with an aircraft is specifically tied to restrictions placed on the flight due to conflicts. When you issue restrictions to fix a conflict, it indicates that the aircraft needs to adjust its route or altitude to safely navigate around another traffic, violation of airspace, or other obstructions. The red W signifies that the flight is under some form of restriction, and when those restrictions are issued, you are required to acknowledge their implications by lining it out.

In contrast, if an aircraft requests a different route, you may reassess the situation, but that doesn't necessitate lining out a red W unless it directly relates to resolving a conflict. Similarly, a change in the aircraft's altitude does not automatically invalidate the restrictions or require the red W to be lined out unless it contributes to easing the conflict. Lastly, if no restrictions apply, it is not the scenario in which a red W would typically be lined out, as it would not be under any contradiction in flight operations.