Updated on 28 October 2019, the following change was made to the Infinite Flight ATC Manual regarding the ‘on guard’ procedures for high elevation airports.
9.1.1 – Controllers must always check what frequency the aircraft is tuned into before sending an on guard message in case they are with another tower/radar facility. In addition, if the airport elevation is deemed significant, controllers may elect to take this into account (e.g. with an airport elevation of 5000ft, controllers may adjust the ‘on guard’ altitude/FL by 5000ft/FL50 respectively). […]
There was some confusion previously among some previously as to whether airport elevation was taken into account.
References: “9.1.1” of the Infinite Flight ATC Manual