I couldn't find any image to show you what I'm talking about but on many aircraft there are markings around the static ports and the pitot probes that say RVSM Critical Area. What is this?
Aircraft need to know their altitudes to avoid bumping into each other in the vertical dimension. The default instrument for this is a pressure altimeter. A pressure altimeter needs access to a static port.
RVSM stands for Reduced Vertical Separation Minima. Basically, aircraft above a certain altitude (29,000 ft) used to be kept further apart from each other vertically (2000 ft) since the accuracy of the pressure altimeters degrades with altitude.
Over the years, as traffic density increased there was a need to pack aircraft closer vertically (1000 ft). This is possible if you have a special more accurate altimeter system in all aircraft and these ops are RVSM (late 1990's and early 2000's).
Now, in order to guarantee that this system works as designed several points need to be satisfied one of which is the integrity of that static port I mentioned earlier. Static ports can be sensitive to external air flow and hence any variation / disturbance / abnormality in the area immediately adjacent to them on the fuselage must be avoided whenever you want to use the aircraft for RVSM operations.
And hence the markings to serve as a guide to all involved in aircraft ops about which is the area to worry about. e.g. Any imperfection in that region will have to be dealt with immediately. And worst case the aircraft would not be capable of flying in RVSM operations till any problems are fixed.
The RVSM critical areas must be inspected prior to every flight to ensure they are free from debris, deformation, or other damage. This is to ensure the altimetric accuracy of the pitot/static system for operation in RVSM airspace. This inspection is typically done by a pilot during their pre-flight walk-around.