That's a grey area I'm afraid. The Reliant is Miranda-class, and that ship class could be anything from science, patrol, escort to a supply ship (later on). The Miranda-class was later replaced by the Nebula-class, and the reason these vessels could either be science or tactical vessels is the pod they had outfitted on the arms moving upward from the warp nacelles, could either be a sensor pod, or a tactical pod containing photon launchers etc. I guess whether you'd call it a frigate or destroyer would depend on the configuration. Several Miranda-class vessels were assisting the attack on the Borg at Wolf 359, but a Miranda-class ship that was encountered by the Enterprise-D (TNG season 2), the U.S.S. Lantree, was a supply ship. So... your guess is as good as mine.



Looking at this from a production perspective, they obviously had a limited amount of models available and built, so they reused them. Picards first command, the Constellation-class Stargazer, was essentially a modified Constitution/Miranda class. The precursor to the Miranda-class was the Soyuz-class which was also encountered by the Enterprise-D. Essentially the same configuration, but more phase cannons (similar to what the U.S.S. Kelvin had in the first of the JJ Trek movies).



Hope all that blarb made sense!