box3d_extent — A box composed of x min, ymin, zmin, xmax, ymax, zmax. Often used to return the extent of a geometry.
box3d_extent is a data type returned by ST_Extent. In versions prior to PostGIS 1.4, ST_Extent would return a box2d.
This section lists the automatic as well as explicit casts allowed for this data type
Section 8.5, “PostGIS Box Functions”