When STII came out in 1982 there was speculation that the Reliant was the new NCL being introduced in Expansion #2 which was just released around that time. But we thought it may have been a NCLX given that it was "movie Era" technology. The external photon pod on the Reliant solves one of the most pressing problems of Fed ship design...the heavy "direct fire" weapons limited to the FA arc. Reliant solves this HUGE PROBLEM by expanding it to FA / RA thus depriving your opponent of the preference to move on your rear where you are least defended. This idea is not new to Trek in that the Klingon ships in TMP where they were introduced had a RA heavy direct fire weapon. ( torpedo ). Tactically this as a new feature in Fed ship design would be a Godsend.
From what little I remember hearing about the Reliant when it was approved for STII, it's design was signed upside down, thus making the designers feel that the design was approved as presented inverse on top instead of origionally designed with it on the bottom with the weapon's pod origionally intended on the underside of the ship. So this says then that the Reliant was origionally intended as designed to have it's torp pods below and it's warp engines on top like the Enterprise. But turning the design upside down definately makes it stand out more in the threatre in contrast to the Enterprise with engines on top, and torps below. I know they were concerned that the ship needed to look different from the Enterprise so that the movie audience wouldn't be confused as to which ship they were looking at on the screen during the combat scenes. And the external torpedo pod may very well have been a smaller secondary hull. But given it's scale size in relation to the similar torpedo bay on the Enterprise's dorsal, I would seriously rule out that it was intended for anything else than a external photon torpedo bay given it's rather small size.
So my answer is Torp bay...and not intended for really anything else.
