A box type is defined by the three side dimensions of a box and
its placement constraints. A box type may contain one or more
boxes of the same dimensions and placement constraints. A box
set is characterized as homogeneous if it only contains one box type.
A box set is considered as a weakly heterogeneous one if it only
contains a few box types, and each box type includes a relatively
large number of boxes. On the other hand, a box set is regarded
as strongly heterogeneous if it includes many box types, and each
box type only contains a few boxes.