Harmon Rabb is referred to as Lieutenant Commander and bears the rank insignia of a Lieutenant Commander. At this point in the series he doesn't hold the rank of LCDR, but rather that of Lieutenant. He is promoted to Lieutenant Commander in Episode #1.13, "Defensive Action".
In the opening scene, the acting JAG Commander Lindsey refers to Lt. Harm as Commander and makes references to Harm's actions of representing CAG and parachuting into Bosnia. Lt. Harm doesn't get promoted until episode #13, "Defensive Action". Harm is notified by Acting JAG Lindsey and is surprised by his unexpected promotion to Lt Commander. During the episode he represents the CAG during court martial. By episodes end Harm has parachuted into Bosnia to find the witness and the evidence to save the CAG's military career.