Pressure ratio
Pressure ratio compares outlet pressure to inlet pressure. It is not the same as boost gauge pressure because it includes atmospheric pressure and losses. Higher boost means a higher pressure ratio, but altitude, filter restriction and intercooler pressure drop affect the real number.
Corrected airflow
The flow axis tells you how much air the compressor can move. This is closely linked to horsepower potential. A turbo that sits in a strong efficiency island at your target airflow and boost will usually make cooler charge air and require less shaft speed than one operating near the edge.
Surge and choke
The surge line is the left boundary where airflow is too low for the pressure being produced. Surge can make the turbo unstable and noisy and may damage parts. Choke is the high-flow edge where the compressor cannot efficiently move more air. A good match keeps the engine’s operating points away from both extremes.
Maps are guides, not gospel
Compressor maps help, but they are not the whole build. Turbine choice, manifold design, cams, intercooler, exhaust and tuning still decide how the car behaves.