@Name_Entry if you're playing in a server that has CItybound but does not have the terraforge mod then yes, you could go in singleplayer, craft the terraformer and bring it to the server, you can access server even if you have the terragorge mod subscribed nothing bad will happen, you just won't see the terraformer in the terraforge when logged, assuming the server has Citybound installed.

Alright, thanks to detailed explanation. As I understand about it,
Can access server even if server doesn't have mod
Can't see recipe if server doesn't have mod
Can make it in singleplayer and bring it to the server even if server doesn't have mod
right?

Yes it just adds just recipes no new assets, the terraformer is already present in the Citybound mod, client-side in this sense.
Although to be used in a server the server should have this mod present or the players won't see the terraformer in the starforge even if they have this mod installed (but they could spawn it in single player and bring it to the server if it has the citybound mod)

This mod is client-side, if Citybound already installed both server and client.
I guess this mod just add recipe, that already exist in Citybound mod.

Really helpful for consistency, considering that the terraformer for the planet already exists in Citybound.

If you make a patch that prevents Citybound planets from naturally spawning, (with this patch as a requirement, obviously) it would make the Populated planet feel much more special & turn it into an endgame reward.