The History of the Mark Classification System
Whenever a starship is designed, whether civillian or military, personal or commercial, all of them fall under a specific classification. There are two types of classifications available to designers. One dates back to the days of sailing vessels. Simply called classification, this means of categorizing defines a vessel's function; either cruiser or freighter or warship... etal.
But the other is realatively new. Dating back to the birth of the Federation, the Mark Classification System is a deceptivly complicated system relating the ship's exterior design and its interior components with contemporaries and predecessors.
Captain Marcus Alguins, a human in comand of the S.S. Eagle during the Romulan-Earth War, had noticed relationships between his ship and others within the same fleet. He wrote the similarities down and also the differences and published them under an assumed name, Mark. The term 'mark' had been used to identify the differences between ships within the same class, but that was proving to be too much work. Simply put, he started with the first warp-capable ship as Mark-1 and went from there.
At Marcus' time, there weren't many external differences, most looking like the S.S. Valiant, but there was enough internal and system differences that he was able to quantify labling fourteen different classifications. The United Earth Space Navy, UESpacy, was skeptical of using his system of classification due to the increased governmental control that would be necessary to classify all the new ship that were being constructed at the time. But Marcus didn't care about that, he just published what he thought was a continuing trend in interstellar starship design.
The lead techinician for the newly founded Federation re-read Marcus' paper and decided to incorporated the system within the Federation's new military force, Starfleet. Up to this point, governmental control was seen as too much power in the hands of someone who could go corrupt. The threat was, at the time, too great to even mention. But the Federation knew that the only way to get things accomplished when dealing with an interstellar community encorporated with multiple species was to get more involved with the actions of the people without interfering with them. So Comodore Jeff Imada, Lead Starfleet Engineer, authorized the use of the Mark System.
Before the Federation, ships were built by private contractors with only money to gain. Often times they wouldn't even build a similar ship in a similar fashion which lead to the increased number of classifications. When Starfleet took over, most of those private contractors dropped out of sight leaving Starfleet to design and build its own ships. Starfleet Corps of Engineers was given more power than just repairing old ships and bases.
Captain Marcus was not forgotten, though. He was recruited into the Corps to help them design a new ship: one that would usher in the new era of Federation alliance and Starfleet exploration. With his help, the Daedalus class was born at Utopia Planitia, a new shipyard under total military control. Truly an era of new starship design had arrived with the increased technological understanding of the other member worlds. The ship was designed to be comfortable and not utilitarian: homely and not militaristic. This lead to the largest construction of its time and lead to the most spacious of ships in the fleet. Over twenty ships were ordered, but only fifteen were ever constructed.
Even though the class lasted nearly forty years, the stage was set and the Mark Classification System was carved in stone. The Daedalus class was retired in August of 2196. But oddly enough, the next level in classification wouldn't occur until a new computer system was invented nearly forty years after that.
Over the last few centuries, though, the MCS has come to define major changes in the exterior design of the ship. From the time of the refit Constitution class, many ships were designed, but thier core systems were standardized and localized. Not much changed in the way of systems for quite a while, and when they did, Starfleet generalized them and standardized them to all ships in the fleet. This meant that the only differences were in the exterior of the ship.
Below is the Classification System starting with the Daedalus class.
Official Mark Classification System
Mk-15 Spherical Primary Hull (Daedalus)
Mk-16 Flat Saucer Primary Hull / Duotronic Computer (Constitution)
Mk-17 One Nacelle / No Secondary Hull (Hermes)
Mk-18 New Nacelle, Hull, Computer (Constitution Refit)
Mk-19 One Nacelle or Two Matted / New Deflector (Hermes Refit)
Mk-20 Horizontal Pylon arrangement (Federation)
Mk-21 Four Nacelles / No Secondary Hull (Constellation)
Mk-22 No Secondary Hull (Miranda)
Mk-23 Bottom Pylon arrangement (Belknap)
Mk-24 Non-Round Secondary Hull (Menahga) Battlecruiser Testbed (F)
Mk-25 New Nacelle, Hull, Sensors (Oberth)
Mk-26 New Nacelle (Excelsior) Transwarp Testbed (F) Battleship Testbed (A)
Mk-27 Incomplete Hull (Akayazi)
Mk-28 Downward Nacelle / Secondary Hull (Shi-Kahr)
Mk-29 Four Nacelles / Secondary Hull (Apollo)
Mk-30 New Nacelles / Separable Hull (Ambassador)
Mk-31 Stretched Hull (Alaska)
Mk-32 No Secondary Hull / New Nacelle (Antares)
Mk-33 Elevated Pylon (Challenger)
Mk-34 New Nacelle / Separable Hull / Elliptical Primary Hull (Galaxy)
Mk-35 Three Nacelles / No Secondary Hull (Cheyenne)
Mk-36 Extra Large Shuttles (Danube / Runabout)
Mk-37 New Nacelle / No Secondary Hull (Defiant) Warship Testbed (A)
Mk-38 Variable Warp Geometry / Fused Hulls / New Nacelle (Intrepid)
Mk-39 Incomplete Secondary Hull / New Propulsion / Catamaran (Akira)
Mk-40 Incomplete Secondary Hull (Steamrunner)
Mk-41 Thin Hull / Sectioned Primary Hull (Majestic)
Mk-42 Fused Primary and Secondary Hull / Elliptical Saucer (Sovereign)
Mk-43 Multi Vector Attackship / New Warp Propulsion (Prometheus)