This International Standard defines terms relating to hot cathode ionization vacuum gauges, and specifies which parameters are given by manufacturers of hot cathode ionization gauges and which measurement uncertainties have to be considered when operating these gauges. The reasons for this are as follows. This International Standard updates some terms and definitions given in ISO3529-3:1981. This International Standard specifies information for suitable laboratories to correctly calibrate vacuum gauges under high and ultra-high vacua, since ionization gauges with hot cathodes are often used as reference standards. This information consists of the relevant parameters and characteristics suitable for quotation in manufacturers\' instructions to users employing ionization gauges for traceable measurement of pressure under high or ultra-high vacua. This International Standard also lists those uncertainties associated with the measurement of pressure by the ionization gauge, which are known to be significant, and gives guidelines on how to evaluate them. It is possible that the list is not comprehensive for some current or future vacuum gauges. This International Standard complements ISO/TS3567 and ISO/TS27893 when using ionization gauges as reference standards.