The Terraformer expands the usable building space of a planet. It becomes important on developed colonies when occupied fields approach the planet limit and new buildings can no longer be placed comfortably.
What exactly does it increase?
It increases the planet field limit according to the building definition stored in the selected universe catalog. Inter-War uses immutable per-universe catalog snapshots, so exact costs, requirements and field gains are rules of that universe rather than a value that should be guessed from another game version.
Does it reroll colony size?
No. The colony’s base size is determined by colonization. The Terraformer later expands usable capacity on that planet; it does not reroll the original colony profile.
When should you build it?
When free fields are running out but you still need higher mines, power, storages, research or other buildings. Large planets can delay this point; small colonies reach it much earlier.
Why is there no universal field number here?
Building definitions are part of the universe catalog. The authoritative value is the next-level preview and requirement data shown in your current universe.
FAQ — Terraformer
Does it produce resources?
No. Its purpose is building space.
Does it change planet temperature?
No. Temperature and the original colony profile are separate parameters.
Related FAQ: How do resource storages work? · How is resource production per hour calculated?