Every airplane has that. A fully computerized cockpit will have that. Its a feature. It has to be manually set because there is no way to know what the referen

Every airplane has that. A fully computerized cockpit will have that. Its a feature. It has to be manually set because there is no way to know what the reference pressure should be for altitude while you are in the air (the reference isn't necessarily the true sea level pressure since that changes constantly, it is what the SLP was at the time the reference was set). Using a non-pressure altimeter you can normally figure out what true SLP is, but the idea is that you want all planes using the same altimeter readings, even if it is not strictly correct. All aircraft in a given area are told the reference by the controller when they enter the control area.