It might be confusing for some as to why so many gauges come with 4 different possible readings. Basically, different situations have different conventions, but usually, PSI (pounds per square inch) is the dominant unit. Similar to psi is kPa, where Pa is pascal (Newtons per square inch) and k is kilo (so a kPa = Pa/1000). The unit bar expands on this still, where 1 bar = 100kPa. This measurement is used because it is the approximate pressure in our atmosphere. Finally, there is Kg/cm², yet another way to measure the same thing—in this case it’s the kilograms per square centimeter (kind of like the metric version of psi). At the end of the day, it depends on which measurement you need.