The bar and its submultiples are accepted for limited use in meteorology only. It is not accepted for use in the U.S. for other applications, e.g., as the unit of fluid pressure in pipes and containers. The appropriate SI multiples, e.g., kilopascal or megapascal, should be used instead. This acceptability for limited use applies to millibars under their own name, and does not change the fact that kilopascals are to be preferred over millibars. Millibars flying under their true colors are obsolete, but still better than hectopascals.
Millibars are obsolete units.
It is the barye which is the coherent cgs unit of pressure, equal to one dyne per square centimetre. There is finally increasing pressure to use the International System of Units and abandon old units such as these time-worn millibars. But the scheme to hang on to these obsolete units by disguising them in a pseudo SI name is a crackpot idea. Millibars are outdated, but at least they were once proper units of pressure. I said above that U.S. Federal Meteorological Handbook Number 1 uses hectopascals.