From AMS Glossary
(Or mandatory surface.) One of several constant-pressure levels in the atmosphere for which a complete evaluation of data derived from upper-air observations is required.
Currently the mandatory pressure levels are 1000 mb, 925 mb, 850 mb, 700 mb, 500 mb, 400 mb, 300 mb, 250 mb, 200 mb, 150 mb, 100 mb, 70 mb, 50 mb, 30 mb, 20 mb, 10 mb, 7 mb, 5 mb, 3 mb, 2 mb, and 1 mb. The radiosonde code has specific blocks reserved for these data. To have a more complete vertical picture, significant levels of radiosonde observations are also evaluated. Until just after World War II, mandatory levels were defined by values of height above sea level.
term edited 9Sept2014