"Hidden" Systems
The simple "Hull Integrity" meter present in SFC, the "hit point bar" as non-SFB players are likely to think of it, is much more than a meter showing total damage. It is a representation of all of the systems on a ship, which do actually exist so that the DAC can work properly. Systems like Hull, Lab, Cargo, Transporters, Tractors, and Impulse Engines do exist in SFC; they are simply represented abstractly by the Hull Integrity meter so as not to clutter the ship display. Each ship does, in fact, have the proper number of each system and each is represented on the internal DAC; you just never see any of this.
This meter displays based on the relative BPV value of each system so, for example, if a Romulan Warbird loses its Type-R launcher a significant portion of the Hull Integrity meter would become depleted. If the ship repairs the launcher, a significant portion would be replenished. In other words, the more valuable a system is the larger "chunk" of the Hull Integrity meter will be gone if that system is destroyed (or be restored if that system is repaired).