Subclasses have been a thing since the old Franz Joseph Schnaubelt Star Fleet Technical Manual. The Constitution-class was only the first of the Heavy Cruiser type, with Bonhomme Richard and Achernar as the next two variants. Unofficial publications fleshed these out usually as Phase II-style upgrades between the TOS ship and the Enterprise-class refit of the first movie. Growing up reading that stuff, it's kind of ingrained in me and I just assume the Soyuz-class is a subclass of the Heavy Frigate, of which Miranda (or Avenger, as it was called in these unofficial books prior to Mike Okuda officially naming the class via a dedication plaque in TNG) was the first type. The Saratoga and Lantree were presumably different subclasses too. Diane Carey's Ship of the Line goes into some detail about the USS Bozeman. I really like the backstory and mission of the ship as she depicts, where it even has flashing police lights (finally seen on-screen on the mini-Salcomes in Star Trek Beyond)
But it's all up to interpretation.
But it's all up to interpretation.
