I thought it was obvious from the context, but I guess I should have been more specific to avoid misunderstandings.
Not just you. I've seen it mentioned a couple times, but it's always been without it's descriptor. Which, as I don't play with champions, never realized there were three others.
They are just like the human mod-fort, only different. :p
All of them have the same number of small and large slots, hp, hull type, speed, radar dampening... but that's pretty much where they stop being the same. Their main weapons are different, and of the champion mod unlocks, some can only be used on their racial mod-fort, some on the racial and on the human.
"Firing lane forts" - these excel at being placed directly in the lane that enemy waves charge towards the station:
Spire: main weapon is a really heavy damage Spire beam, range 28,000. Can be equipped with high damage Photon Lances (range 25,000) in the large slots.
Zenith: main weapon is a heavy damage Zenith beam, range 43,000. Can be equipped with high damage Heat Beams (range 20,000) in the large slots.
Lots of other funny things they can be equipped with depending on unlocks (Plasma Siege Cannon, Railcannons, Railclusters etc.), but making a hole in the line of mines leading to the station and placing them in that hole to let attacking enemies fly directly up their powerful beams is quite fun and something for which the normal unlockable forts really don't have an equivalent, not even if you equip it with Heavy Beam Cannons (range 14,000) .
"Bizarro fort"
Neinzul, the cloaking fort that doesn't have any special large slot mods, so probably used bomber or slicer bays, but can be equipped with Insanity Inducers (the budget Botnet fort!), Doom Accelerators (anything that survived the beams of the other forts has got to be damaged...), or Translocators in the small slots.
It is not that the standard Fort marks 1-3 and human mod fort with only tech unlocks are bad - they certainly aren't - it is just that the other three mod forts give you more options, some of which are
very powerful due to how they complement your other capabilities. Without champions enabled, all the human mod fort gives you are
more of some of the capabilities you already have in your defenses - more missiles, more lasers, more of the basic fort offense (like a mark 1 fort)... whatever. (And if you have champions enabled some of their modules can also be used on the human mod-fort, extending its options, but the best remain specific to the individual other race hull).