Earlier quoted context omitted.
In a voxel editor you edit a 3D world using the smallest unit as cubes/spheres/etc. in 3D space. This is a 2D pixel editor with "stacks" of sprites.
Isn't the stack then just the third dimension of the original two dimensions?
However, not every engine supports 3d models / voxels, so sprite stacking is commonly used in combination with engines that only support 2d, by simply drawing each layer on top of the previous one but one pixel higher on the y axis.