[Extract from the SHIP NAVIGATOR HANDBOOK]

There is no official classification of spacecraft that would be accepted across the entirety of human space. However, the Ecumenic Identification System (EIS) provide a handy set of guidelines according to which ships can be identified. These guidelines will be followed by your orrery displays unless they are set to a regional standard such as the Standardized Solar System Identification or the Eloran Identification And Tracking Table.

Surface dwellers often use maritime terms such as cruiser, carrier, clipper, etc...to classify ships. While not wrong per se (a few regional classification systems use the analogy) this is not very practical. First and foremost because such terms may change depending on culture and tradition and second because space, as you may have noticed, isn't water. Spaceships evolve in a three-dimensional environment and obey constraints (in terms of delta-v, for instance) that do not apply to maritime ships.

The first and most basic classification operated by the EIS is one of capacity, with four levels :

• Down-dwellers are ships that can only operate inside a gravity well: this mostly applies to landers and orbital tugs. Down-dwellers are defined by limited delta-v that does not allow them to reach interplanetary destinations on their own power within reasonable timeframes. The category ranges from satellites to heavy orbital barges.
  
• Up-dwellers are ships that can operate beyond a gravity well but cannot embark on interstellar journeys and are as such limited to interplanetary operations. This category is somewhat under-populated because interplanetary ships are often FTL-equipped but there are notable exceptions in deep space mining operations. Up-dwellers that are used to link stars in binary and trinary system blur the line with the upper categories.
  
• Travellers are ships that are capable of reaching close interstellar targets within a reasonable timeframe: in that regard, "close" means "the ten closest stars" and "reasonable timeframe" means "within six months". You'll notice that the definition of a Traveller is relative to the star density in a given region. In very dense regions such as globular clusters, non-FTL ships equipped with NAFAL drives would theoretically belong to this category.
  
• Vagrants are ships that are routinely capable of unassisted long-range interstellar travel, the somewhat arbitrary criteria being the capacity of reaching the one thousand closest stars within one to three geometry translations. Much like with the Traveller classification this is relative to the local stellar density. 

Note that this part of the EIS was created in the first years of interstellar travel: as computing power increased over time the border between Traveller and Vagrant has become blurrier and blurrier. This is especially true when accounting for station-assisted translations which weren't widespread when the EIS was first established.

The second classification in the EIS system is one of role which determines the three-letter prefix attached to every ship signature on a standard orrery. Roles are relative and are not fixed: ships generally self-declare their role through their beacon.

• LDR: lander. A ship with the ability to reliably perform surface to orbit and orbit to surface travels. Ships that can make landfall once without the ability to reach orbit again do not belong to this category.
  
• TUG: as the prefix indicates, a ship dedicated to carrying other ships, cargo or personnel between orbits. By far the most common ship type in terms of numbers.
  
• CRG: a ship that can carry more than half its hull volume in cargo. This is the most common ship type in terms of tonnage.
  
• PAX: a vessel that is dedicated to carrying passengers temporarily, i.e that rotates most of its passengers between halts.
  
• EXP: an exploration ship. This is a new addition to the EIS and is exclusively self-reported as there is no obvious way of identifying such a vessel.
  
• OPS: a very broad category dedicated to ships that fill various functions but cannot fall in any other category.
  
• COR: courier/messenger ship. 
• MIN/CST: mining or construction vessel.
  
• MIG: "migrant ship" i.e a colonisation spaceship. Often meant to be sacrificed during landfall.
  
• MIY: military ship equipped with ship-to-ship or ship-to-surface weapons.
  
• PAO: patrol ship, generally not armed.
  
• UNI: non-identified, or class not self-reported.
  
• 000: non-human vessel. 
• XXX: discrepancy observed between self-reported class and observable criteria such as thermal signature, radar cross-section, trajectory or visual appearance.
  

Ship illustration by Manchu for Poul Anderson's Tau Zero. All rights reserved.