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UPS classification code
IEC 62040-3 Standard classifies UPS performance by 8-digit code:

• The first three digits classify output dependency of input mains in normal operation:
  
  • VFI – Voltage and Frequency Independent
  • VI – Voltage Independent
  • VFD – Voltage and FrequencyDependant
• The next two digits refer to output waveform:
  
  • S – Sinusoidal for linear and non-linear loads (THDV<8%)
  • X – Non sinusoidal with non-linear loads
  • Y- Non-sinusoidal.
• The last three digits refer to output dynamic performance (including transients during transfer switch operation).

[http://www.upsonnet.com/UPS-Glossary/]