FAR 91.205(b)(8) says that you need a manifold pressure gauge for each "altitude engine", whatever that means.
What's an "altitude" engine?
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An altitude engine is a type of engine that uses a turbocharger (and less often, a supercharger) to generate sea-level pressure in its intake manifold, up to its critical altitude, at which point power will start to fall off.
Non-altitude engines will lose power as they climb because the ambient air pressure drops off at a rate of about 1" Hg per 1,000 ft of altitude (up to 10,000ft).
The altitude engine allows aircraft to fly at higher altitudes at full power, and increases the aircraft's speed and efficiency, because true airspeed increases with altitude. It also allows an aircraft under IFR to fly at higher MEAs or under VFR over higher terrain, and allows the pilot to choose a favorable altitude for weather.
The first kind of altitude engine is called a turbocharged engine, which can generate more intake manifold pressure than at sea level. For example, a Mooney Bravo M20M can generate 36" of manifold pressure, and can generate at least 30" of manifold pressure up to its critical altitude of 22,000ft.
The other kind of altitude engine is called a turbo-normalizer, which develops a maximum of sea level pressure: 30”Hg or 1 Bar. These engines will perform as well at altitude (up to their critical altitude) as they do at sea level.
A turbo-normalized engine is considered less taxing on the engine than a turbocharged engine because the former doesn't develop above sea-level pressure or high turbine-inlet temperatures. The turbo-normalized engine is also easier to fly than a turbocharged engine, since it never develops more power than would be developed at sea level.
All altitude engines also typically have a controllable-pitch prop, and develop more than 200 horsepower, so they would require both a complex aircraft endorsement and a high-performance endorsement. See 14 CFR Part 61.31.
FAR 1 says that an altitude engine is
a reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude.
Basically, if you can develop sea level power at an altitude higher than sea level, you've got an altitude engine. Generally, this means you're flying with a boosted engine (whether the boosting comes from a turbo, a supercharger, or something else).
An altitude engine is one that is either supercharged or turbocharged. It creates its own level of density altitude inside its INTAKE manifold. This makes the engine produce more power and more consistent power regardless of the atmospheric/environmental actual density altitude.
The difference between supercharging and turbocharging is the method you generate the higher amount of pressure (or LOWER DENSITY-ALTITUDE) inside your intake manifold.
Turbochargers have an impeller fan just past your EXHAUST manifold that is spun by the pressure of the exhaust gasses escaping your cylinders before reaching the muffler. That is connected to an impeller located between your INTAKE manifold and the fresh air cleaner/filter. That impeller drives or boosts the amount of air going into your engine, increasing its pressure.
Superchargers are air pumps (think industrial pump and not bicycle nor shop pump) that are attached directly to your INTAKE manifold. They are connected to your crankshaft usually via a system of belts and clutches just like your alternator, water pump, air conditioning pump, hydraulic pump, or engine driven fuel pump (depending on the model).
Turbochargers are typically smaller in size and lighter in weight. But, they are exposed to very high heat and RPMs promoting more wear and tear.
Superchargers typically take up much more space and weight. But, they are only exposed to ambient engine heat and their own internal friction. They spin at lower RPMs to produce the amount of air volume pumped or boosted into the INTAKE manifold to increase pressure. The mechanically driven pump theoretically has less wear and tear.
Either one boosts the pressure in your INTAKE manifold beyond what is in the ambient atmosphere. Air is being blown/forced into your engine as opposed to having the engine suck in the air it needs. Like a water hose instead of a straw or a ventilator instead of your lungs, your engine is being force fed.
Either way, it is not the increased pressure in your INTAKE manifold which is truly important. Nor is it the increased volume of air. It is the increased amount of oxygen molecules provided by either INTAKE system for combustion with the fuel in your cylinders that makes the difference. Air density (or to be more precise, oxygen density because the percentage of oxygen will be consistent regardless of pressure or volume) is the true goal of both systems. Since both systems will cause the air to heat up adiabatically (increasing the pressure of and/or decreasing the volume or available space for a gas will cause an increase in temperature), that is why many systems will incorporate an inter cooler (air cooler located between the turbo/supercharger and the INTAKE manifold) into the INTAKE system. The cooler the air, the greater the air density.
THE DENSER THE AIR/OXYGEN-IN-THE-AIR, THE MORE POWER/THRUST YOU GET FROM YOUR ENGINE.
Sorry for the long explanation. ;)