Air charge is the percentage of air within the cylinders compared to how much the cylinders would hold at atmospheric pressure. Anything above 100% air charge is under boost. The ECU uses it as the primary axis for most lookup tables in place of load on other platforms. Rather than targeting a set amount of boost, air charge is targeted instead so that pressure, temperature, airflow, and engine speed are all taken into account.
Boost pressure and air charge are closely related, but they are not the same. Boost pressure is a measurement of the force the air is exerting on it's surrounding space compared to atmospheric, whereas air charge is a measurement of the amount of air within the cylinder. Both are increased by the turbocharger, but boost pressure does not take into account the temperature of the air nor the RPM and flow of the engine and thus cannot be used to target a certain amount of torque output.
Combining temperature with pressure lets the ECU figure out the density of the intake air. Some ECUs work with just this calculation combined with the engine speed for a speed density tune. However, the ECU integrates flow into the equation as well, which is notably something that is measured over time.
That's where the MAF sensor comes into play. It tells us, via the AFM flow voltage table, how many grams of air was pulled into the intake over a certain period of time. But wait, that's mass! Can't we just use that? No, because it is uncompressed mass that will have to sit behind the air that is already compressed up ahead. The air flowing through the system does not flow linearly and it won't be representative of what's flowing into the cylinder itself.
The ECU combines this pre-turbo mass airflow with the post-intercooler air density to find out how much air is sitting just after the throttle plate and before the cylinders, and then divides that by the engine RPMs to make it an immediate measurement of how much air is in the cylinder. This measurement is then converted into a percentage where 100% equals a full cylinder at atmospheric pressure.
Targeting air charge and using it as an index for most tables is the primary means by which the Bosch ECU safely makes a specified amount of torque in any environmental condition.