Commander Third Grade Edit
The highest grade of the Commander rank. Starship XOs (Executive Officers, Second-In-Commands) are usually Commanders Third Grade. Lower-grade Commanders can technically be given the XO title, although it is more common for the XO to receive an honorary and temporary Commander Third Grade rank to accompany the position.
Commander Second Grade Edit
Intermediate grade of Commander rank. Generally not a rank held for extended periods of time, as it is seen as a stepping-stone to Third Grade. Alternatively considered to be a "dump rank" for the placement of competent officers otherwise unsuited to higher command positions. The same general view applies to Second Grade ranks for Captains, Lieutenants and other officers and crew.
Commander First Grade Edit
The lowest grade of the Commander rank. Inexperienced, untested, freshly-promoted or freshly-graduated officers are generally placed in this rank in order to allow them to gain command experience without having to shoulder much command responsibility.