This gets complicated and is a great example of government bureaucracies at their worst. It all came about because of the exploding OE Firestone tires on the Ford Explorer (hence it's nickname of "Ford Exploder"). Firestone recalled over 6 million tired in 2000 because of this. Ironically, it was not Firestone's fault. Ford had listed a very low inflation rate for the tires to make the ride on these heavy, full-frame truck-based SUVs more compliant for the soccer mons who were the primary market. That made the tires flex more, heat up more quickly, and hot tires tend to fail. So in 2000, the NHTSA began working with congress to craft legislation requiring TPMS.
The legislation (The TREAD Act) did not get finalized until April 2005. It phased-in TPMS requiring 20% of all MY2006 vehicles to have it, 70% for MY2007, and 100% for MY2008. The law also required a dedicated TPMS system (i.e. using sensors mounted on each wheel, not an ESP-based system). The system was not required to report individual tire information to the driver; it only needed to alert the driver if any tire needs attention.
MBZ already had a software (ESP) based system available. In fact, many MBZ cars prior to MY2008 without TPMS actually have the software and it can be enabled via SDS. The problem is, once the legislation was finalized, they found it would not meet the law and by April 2005, MBZ had already started building MY2006 cars so they had to scramble. The easiest thing to do for this short notice was install sensors in the wheels and a single radio receiver module that could then communicate over the CAN bus and alert the drive on the instrument cluster. That system could actually get all the individual pressures, but it had no idea which tire was which so all you had was a generic warning, This is "low-line" TPMS. Later, when they had more time to engineer it, they added multiple antennas to triangulate wheel positions and then they had the "high-line" TPMS.
An interesting note is that many (non-MBZ) cars that have TPMS with individual pressure displays require using a special tool to program the car when tires are rotated. Basically each TPMS sensor has its onw "channel" and the car's system has to be told which tire location uses which channel. MBZ's use of multiple antennas (and they also facto in ESP information) allows for tire rotation without any programming.. Chrysler also uses this system (as it was designed by then parent-company Daimler). Many manufacturers still use system like MBZ's "low-line" where individual pressures cannot be displayed. Some are not even programmed to "understand" which is a correct pressure. instead, it's up tot he owner to technician to properly inflate all tires, then press a button (or use a special programming tool) to set the calibration.
BTW, MBZ's calibration is set from the factory but you can change it. For example, you may use the "100MPH+" inflation specs instead of the standard, which is usually a 4-5psi difference. To set your own calibration, inflate the tires, scroll to the TPMS screen on the cluster and press and hold the trip reset button until it beeps.
