From the International Electrotechnical Commission webstore:
“IEC 61643-311:2013 is applicable to gas discharge tubes (GDT) used for overvoltage protection in telecommunications, signaling and low-voltage power distribution networks with nominal system voltages up to 1,000 V (r.m.s.) AC and 1,500 V DC. They are defined as a gap, or several gaps, with two or three metal electrodes hermetically sealed so that gas mixture and pressure are under control. They are designed to protect apparatus or personnel, or both, from high transient voltages.
This standard contains a series of test criteria, test methods and test circuits for determining the electrical characteristic of GDTs [with] two or three electrodes. This standard does not specify requirements applicable to complete surge protective devices, nor does it specify total requirements for GDTs employed within electronic devices where precise coordination between GDT performance and surge protective device withstand capability is highly critical.”