Not all naval bases are created for the same reason, excluding naval depots there are several broad types. Often its not a case of certain types having certain facilities so much as cetain types will have some of their facilities more developed than others. The main types are:
Alpha Base: An advanced or forward base, a border outpost. Typically found along the Imperial border an Alpha Base will have an extensive intelligence section and often a number of small recon starships.
Beta Base: A naval base focused on building ships. In addition to the base's shipyards there will be extensive industry either nearby on the host planet or on a number surrounding systems (with regular inbound shipments).
Gamma Base: A naval base focused on research. Similar to an Imperial Research Station but (a) exclusively for the Imperial Navy, and (b) more for application development than pure research.
Delta Base: A diplomatic outpost. Found either on (a) new member worlds to facilitate integration of a local military into the Imperial ranks, (b) on culturally mature member worlds with significant planetary or subsector forces, or (c) in troubled regions (diplomacy is just another weapon in the Imperial arsenal).
Epsilon Base: A fleet home port. When a fleet has been based out of a specific system for a length of time a logistical support operation evolves. There are expanded ship repair facilities, supply stockpiles, and extensive staff facilities. The size of the local R&R facilities are also boosted.
Sigma Base: A supply dump. While it may have normal day-to-day operations, its primary purpose is to stockpile supplies, spare parts, etc.
Theta Base: A training base. Many (but not all) Theta Bases are Naval Academies.
Zeta Base: Some naval bases have served their purpose and have been scaled down, others are built to "show the flag" but little else. Such low-priority bases are Zeta Bases.
Naval bases use a short UNP (Universal Naval base Profile) in the form X-YZ ... where X is the type, Y is the size, and Z is a qualifier. A UNP is generated as follows:
An Epsilon base is designated by the Referee only (not random)
Treat rolls less than 1 as 1, and greater than 6 as 6
Bare in mind that the type represents the primary purpose. It is entirely possible for a G-4 to have a few recon ships and a construction slip as well. (Author's note: I did consider expanding out the profile rather than use the 'qualifier' code but I wanted to capture the essence of the base rather than describe it fully.)