This Standard is a guide for the understanding and application of the International System of Units (Système international d'unités), for which the universal abbreviation is SI. Information is included on quantities, SI units, and the recommended application of SI multiples and submultiples. Up-to-date terminology for quantities and conversion factors that relate the SI to other systems of units is also provided. The conversion and rounding of data and dual dimensioning are also addressed.
In this Standard, shall indicates a a mandatory requirement; should indicates a recommendation or that which is advised but not mandatory; and may indicates an advisory or optional statement. Notes accompanying clauses do not include mandatory or alternative requirements. The purpose of a note accompanying a clause is to separate from the text explanatory or informative material that is not properly a part of the requirements of this Standard. Notes to tables and figures are considered part of the table or figure and may be written as mandatory requirements. Legends to equations and figures are considered to be mandatory requirements. In particular, shall is used whenever there is only one way to comply with the SI standard. Should is used when there are options within the SI, but SI has established a preference. May is used when there are options within the SI and the SI has no preference.