Users requiring more detailed information may access SP 811 and SP 330 online from the Bibliography , or order SP 811 for postal delivery . Information regarding the adoption and maintenance of the SI may be found in the section International aspects of the SI . Some useful definitions A quantity in the general sense is a property ascribed to phenomena, bodies, or substances that can be quantified for, or assigned to, a particular phenomenon, body, or substance. Examples are mass and electric charge. A quantity in the particular sense is a quantifiable or assignable property ascribed to a particular phenomenon, body, or substance.
Examples are the mass of the moon and the electric charge of the proton.
A physical quantity is a quantity that can be used in the mathematical equations of science and technology. A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. Thus, the numerical value of a particular physical quantity depends on the unit in which it is expressed.