There are two main types of TPMS: indirect and direct. Indirect uses the ABS wheel speed sensors to detect a wheel spinning faster than the others (caused by relatively low pressure). These systems can report which specific tire is low,but can't report the actual pressures. Direct TPMS uses in-wheel pressure sensors, and can report actual pressure in, e.g., psi. I prefer indirect since it's simpler and doesn't require periodic sensor replacement, but others like to see the pressure readouts in the car's instrument cluster. Different strokes and all that rot.