Besides initial composition, what critical factor retains a planet's terrestrial classification?

Remaining below a certain mass threshold to avoid capturing vast light gases.

While the starting materials (rock and metal) are essential, retaining the terrestrial label depends heavily on mass constraints, especially when considering planetary formation dynamics. If a world grows too massive, regardless of its initial rocky core, its gravitational strength becomes so dominant that it captures large envelopes of light elements like hydrogen and helium present in the protoplanetary disk. Crossing this critical mass tipping point causes the object to transition into a gas dwarf or mini-Neptune. Therefore, a planet must not only start rocky but also successfully stay under this mass threshold to maintain its classification as a predominantly rocky body.