Commander was a senior officer in the Earth forces who may have commanded a group/wing or Frigate. "Commander" was also used as a form of address used by a Lieutenant-Commander, or even anyone in a command position. Scott Bernard, Roy Fokker, Claudia Grant, Lisa Hayes, Rick Hunter, Tessel, and Olivia Stahl were all addressed and/or referenced as "Commander" at one point.
See Ranks (Earth) for a complete overview of Earth forces ranks.