According to ISO 5599 standard, valve schematic diagrams are designated with numbers (previously letters).

Example of designation: 5/2 monostable valve, solenoid operated with pneumatic assistance (with external pilot supply), spring return:

Distribution valves

1 – supply connection port,
2, 4 – working or outlet connection ports,
3, 5 – outlet to the atmosphere (venting connection ports),
12, 14 – control connection ports,
82, 84 – venting connection ports of supporting valves,
X – pilots external supply,