It depends on the context.
For pneumatic systems, Pounds per square in (PSI) or Bars (or officially, MegaPascals, MPa), where 1 Bar is "Standard" atmospheric pressure at mean sea-level at 20ºC.
For weather, milliBars (mBar), because weather systems are variations of no more than a few tens of mBar above or below the mean, 1 bar, level.
Sound-level calculations use deciBels (logarithms of ratios) of quantities of the incredibly tiny micro-Pascal, which is one-hundred-thousand-millionth of 1 Bar!
The pascal (Pa) is the standard unit of pressure.
mb is a unit of measurement for pressure, 1 mb is 0.001 bar. mb is an abbreviation for millibar 1 bar is roughly atmospheric pressure
milimiters of rain which works out as a liter per squere meterInches or centimeters.
An example would be converting cm into metres, or converting yards into feet, it is just allowing us to use a different unit of measurement.
The rotl is a unit of weight measurement in some countries in the Mediterranean region. While not all Muslims use the rotl to measure weight, many Arab Muslims do. Its value varies from 1 to 5 pounds, depending on the region.
Feet.
a centimeter
A shoe.
You would use CM or INCHES to measure a knife
what unit of measure is thickness og a ballpen
Ounces or milliliters.
A kilometre.
meters
inches
Square footage.
Kilograms.
tons