This Federation cruiser was an attempt to develop advanced shield technologies and apply them to next-generation starship designs. The Mercator Shield Cruiser successfully demonstrated these advanced shielding concepts, but was itself a failure. The Mercator was too expensive and too poorly armed to be considered a ship of the line.
As the economic disadvantage of the design diminished over the years, more Mercators were built. However, they were largley relegated to long-range patrol duties.