In Space Empires IV, moons were placed in the same sector as their parent planet. The name would be the same as the planet, with a letter appended. Eg: Sol III would have a moon of Sol III A. This effect was created by giving the moon object a position setting of "Same As X" in SystemTypes.txt.
In Space Empires V, the same exact "Same As X" setting is available. If you use it for moons, they will get the same letter naming convention as in SE4. There is also a new moon positioning tag available, "Offset X Ring Y." When this is used, the moon is placed randomly in a ring Y hexes from object X. Prior to the 1.58 patch, SE5 did not apply the lettered naming conventions to moons with this type of positioning. The moon of Sol III will be named Sol IV. As of SE5 1.58, moons placed with "Offset X Ring Y" are correctly named with the lettered convention.
In SE4 1.44 or earlier, one side effect of the lack of lettered naming is that it is not immediately obvious that moons are, in fact, moons. This causes many players to assume that there are no moons in the game, though such is not the case.
How to Make Moons Have Lettered Names pre-1.58
If you are playing with a version of SE5 earlier than 1.58, and prefer to have moons named with the SE4-styled lettering, you can edit SystemTypes.txt and change the positioning used for the moons. In the stock data files, the first moon appears in the system type "Standard 2:"
Obj 13 Physical Type := Planet Obj 13 Position := Offset 3 Ring 2 Obj 13 Stellar Abil Type := Normal Planet Obj 13 Specific Stellar Obj Name := Any Obj 13 Size := Tiny Obj 13 Atmosphere := None Obj 13 Composition := Any
Note the Position field. You need to change it to read "Same As 3," like so:
Obj 13 Position := Same As 3
You will need to search for all Planet objects with "Offset" style position and replace them with "Same As." In stock, there are only 2 in "Standard 2" and 3 in "Standard 3." In a mod like FQM, there will be many more occurrences.