What are unofficial SI units?

Unofficial SI units are units of measurement that are not part of the International System of Units (SI) but are commonly used in practice. These may include units like the "bar" for pressure, "liter" for volume, and "angstrom" for length. While they are widely recognized and used in various fields, they lack formal acceptance by the SI standards. Their usage often persists due to convenience or tradition in specific industries or disciplines.