The LHD report is intended to detail any occurrence or vertical deviation of altitude greater than or equal to 300 feet in RVSM airspace. The most common occurrences subject to LHD reporting are: inadequate transfer of traffic between adjacent FIR (Flight Information Region), inadvertent ascents or descents, uncoordinated weather deviations, Resolution Warnings (TCAS) and others.
Briefly, when LHD occurs, an aircraft occupies a space unexpected by the ATC, making the trajectory anticipation made by the ATC no longer correspond to the reality of an aircraft trajectory. Unaware of the existence of the aircraft, the ATC can instruct another aircraft to occupy that position, increasing the risk of aerial collision.
Below are some examples of situations that can be LHD: