Building and terraforming are often treated as the same feature, but they are not.
Building places structures into a world. Terraforming changes the terrain those structures sit on. A true terraforming system may let players dig slopes, flatten land, raise surfaces, prepare farmland, cut roads or otherwise alter the shape of the persistent environment.
Wurm Online remains an obvious benchmark because terrain modification is part of everyday settlement development. Life is Feudal also connected terrain work directly to medieval settlement construction.
This distinction matters for discovery. A player looking for an MMO where they can build a house has many options. A player looking for an MMO where they can reshape the land around a town has far fewer.
That is why SandboxMMORPGs keeps Terraforming separate from Building, Housing and Player Settlements.