A body. Its value at a point is the distance in mm to its surface, below zero inside, so a block that reads a field can read a Solid as one.
An input that takes a Solid also takes a Bounding Box, a Plane, a Lattice, a CAD Face and a Number.
A Solid also fits an input that takes a Number.
The axis-aligned extent of the Solid.
The Solid's signed distance, as a field.
The Solid turned inside out: what was inside is now outside.
Items that are each a Solid. One Solid wired where a list is taken counts as a list of one. A list wired where one Solid is taken runs the block once per item.